There are a couple of threads already related to that question. No, you can't do it yet.

oooh i found a chink in the reaper armour. seriously, i think this is up there with one of my highest priority reaper features i would like to see. and i don't think this would be easy to do. i think probably the vertical piano roll would be even easier.

Actually I think vertical piano roll would be harder. :) There are some piano roll related FRs: etc.

those features discussed in those threads though aren't quite the same not super hard to do. i'm talking click and drag a cell and move it somewhere else so that, say you have 1C as your kick, you can click and drag that cell anywhere you want, and it will remain 1C for your kick, but you can move it down a bucnh of spots so it displays underneath your snare at A1. this completely removes the need to go into your VST and remap your kick to -1B in order to obtain the same result. afaik, right now, all you can do is name cells any way you want, and to get them in teh right order you want you need to map them to that order directly from your VST, which can be really time consuming. and isn't very change on the fly friendly. also, this would allow you to have your drums mapped properly to your e-kit, and also displayed in the order you like in your piano roll, including all your articulations. SONAR has this feature. reaper doesn't have a drum map feature though. which i kind of like, i like that the drums are not separate from the piano roll, but being able to add this feature would be really workflowish. very fast.

here you go, please vote for it.

a "chink" in reapers armor? Im not so sure about that - do any other DAWs have this feature? that being said, it would be a great feature to have, and it would allow you to do different stuff in a custom piano roll without changing the actual notes being triggered, thereby maintaining compatibility with your midi keyboard and other programs if the midi is exported.

yes SONAR has this feature, other DAW's i'm not sure, but i would expect so, since they have separate drum roll sections apart from piano roll, with drum maps that are dully customizable, which makes adding such a feature really simple.

that's why I still use freeware JAZZ++ to edit my MIDI drum patterns etc. You see the drum names look for it at sourceforge