I do my drums through battery 3 like this:
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I originally had FL Studio rewired through to each of the separate channels so I could build drums through FL Studio's step sequencer and have them trigger Battery 3, but it added extra steps to the process and FL Studio's rewire capabilities are screwed up so now I do it like above.
I would love to have a built in step sequencer in reaper. The way above is pretty sweet though, I can just pound in drums through my padkontrol and it generates all the midi for me on the separate tracks. Or I can hand draw them in with the piano roll, which is what I found myself doing in FL studio often anyways.