If I understand what you're wanting to do, there are a lot of drum synth programs which will convert MIDI to AUDIO. The one I use is Toontrack EZ Drummer and it's fantastic, IMO. The way I work is I take the MIDI track from BIAB/RealBand and copy that over into Sonar, then use EZDrummer as the output for the MIDI drum track. I am sure you could, however, set up RealBand to do the same job, using EZ Drummer as a VST. But the inconvenience of that, IMO, is changing VSTs for other instruments to the drum synth, then back again after the drum audio has been rendered.
the core program of EZD is currently on sale for only $29 on
www.sweetwater.com and a few other places. they also have "expansion packs" which are excellent. the Nashville expansion pack is also currently on sale for $39. Nashville is the first expansion pack I always recommend that anyone get - it's more than just "nashville" sound, it can be used in numerous genres.