You can use recording facilities of EZplayer. Load individual tracks from beats you like, on separate EZplayer tracks (I think you have unlimited number of them), then on a new track consolidate all the individual tracks by recording it. You have all this described in EZplayer manual and some tutorials on Youtube. It's very easy - just place EZplayer before your drum sampler in FX chain window. After you have your drum tracks consolidated, you can simply drag it from EZplayer interface to Reaper's tracks which holds your drum sampler. Easy.