How can I ADD guitar feedback?

About guitar feedback... 

Try this: Buy a cheap nasty practice amp, something like a 10 or 15 watt combo with at least an 8" speaker. Pick up a couple of cheap distortion pedals, Boss DS-1 is ideal or cheap Behringer stuff - it doesn't really matter what it sounds like as you won't be recording it. Using a DI box split your guitar signal, with one going to your interface and the other going into the dirt box and then into the cruddy practice amp. With the amp switched off, set up your DAW to record, get your sound right etc and use a set of headphones to monitor through. Now, position the practice amp on a stool or on the desk, whatever will get it as close as possible to your guitar - you want the speaker directed directly at the pickup area. Switch the amp on and start playing with various levels of gain on the amp and pedals until you start getting feedback - chain 2 or more pedals together or use a dirt box and a compressor to get to that point with as little volume as you can. With a speaker 8" or larger really close to your guitar it shouldn't have to be crazy loud before it feeds back. Now adjust your headphone volume so you can hear the click or whatever clearly over the practice amp, and record your parts. If you're anything like myself or all the other guitar players I know, you'll probably have all that stuff taking up space in a spare room already, so likely you'll be able to do it without having to actually spend anything.

Stu: I like this idea and will give it a try.

you can crank up some headphones and put them over the pickup. Send the guitar signal from the DAW back into the headphones.

I find you don't need much for volume but plenty of distortion. Here's an example with I think guitar rig or probably le pou (I can pull the session up but it's old and it doesn't really matter what sim), and then just moving around my monitors to find the nodes. It's about 3 or 4 guitars and later I played around with some parameter modulation for the intro bit. Don't waste your time with feedback plugins... they are weird.

Here's a little over a minute's worth of just standing there making feedback noise with a couple amps and guitars mic'd up. You'll have to overlook the humming single coils. Added a little reverb and delay for fun.

Wow, been gone a few days...I can't even begin to thank you all enough for all the replies. There are some GREAT suggestions and I'm gonna give them a go tonight and see what happens. Someone asked what I was trying to do. I'm a big fan of just about anything from Johnny Cash to The Smiths...but my true love is for the aggressiveness of hardcore, punk and metal. I played bass years ago, but gave it up around the time I got married...had kids shortly thereafter, and now like 12 years later I'm finding time to play again. Anyway...just playing some noisy, heavy stuff...and have a few songs written where a little guitar feedback would add a little life to some parts that feature just drums...like in an intro with a sort of tribal tom beat. Would be really cool to have some feedback fade in until the guitars and bass kick in. So yeah, nothing crazy...not gonna go Eddie Van Halen and hold a drill to my pickups. Just adding a little noise that this software seems to do such a great job of getting rid of. Thanks again for all the suggestions...home recording (and playing guitar for the most part) is pretty new to me, and will certainly be sticking around this forum. Oh yeah...I just made the jump from Garageband to Reaper. Was intrigued after hearing that Reaper can automatically separate an EZ Drummer kit in to their own individual tracks...and now finding so much more that I've been missing with Garageband.

I have one of those Boombox/Rock It/Vibe It thingies headed my way. My interface has a real time monitoring set up and I'll be able to hook the Vibe It directly to the headphone jack of the interface. I don't want to get my hopes up too much, but I'm thinking this could be the shiznizzle. I hope to be able to report back in a couple days.

Here's a trick I use that works phenomenally: I use Zebra but any synth with a sine wave oscillator will do, even the built in Reasynth. Set up three or four tracks with each playing a steady sine wave corresponding to the harmonics of the pitch you wish to sound, e.g.: 1st track - C1 2nd track - C2 3rd track - G2 4th track - C3 .... You can go on for as many harmonics as you like but four is probably enough. ( in case you were wondering). The additive "SpectroBlend" osc in Zebra is perfect for this but as I said earlier any synth will do. Next step is to automate the volume of each harmonic so that they fade in and out, the lower ones being predominant. For example, let's say if the feedback builds up over a couple of measures, first the fundamental would fade in, cross-fading into the first or second overtone briefly, then introduce just the slightest bit of the third overtone towards the end. There's just an example. This could also be done with one or two synths with multiple oscillators being volume-automated separately. Next, feed all those tracks into your amp-sim of choice (e.g. Amplitube for OP). The easiest way to do this in Reaper would be using a folder track. For added authenticity try very briefly introducing a slightly detuned sine wave, I think this makes it sound great! Mind you, I recognize that this style of "micro-editing" may seem mind numbing for some, but also great fun for those OCD among us!

I agree with Stu, the best way is to have DI that splits the signal and takes it to a real amp. This way you can record the feedback sound within the DI signal and you don't need stellar amp or even a microphone. I would feel more unease to use studio monitors for this, also it is easier to get intimate with amp and there is literally a zero latency. Distortion or OD pedal before amp might help a lot. I've also found that wah pedal can really help to excite that feedback. Also some guitars are more prone to feedback than others. Hollowbodies feedback pretty easily, but of course their sound might not be what you are after...