The quickest way is to open up the FX Chain window containing your instrument and go : menue -> options -> build multichannel routing (or the like, not sure about the exact name). Reaper will create a track for each output of the plugin and the correct routing. Configure your drummy to send audio to it's outputs and your good. It's a good idea to save a template with all the related tracks so you can load it in one stroke. Of course you can also do it by hand. It's a matter of setting the amount of track channels you need, creating tracks for the sub-channels and sending the signals via sends/receives. Do the automatic way and study the I/O window of the original track to get an idea what's going on.