You can record the drum track from combi mode if you set it to EXT,(Timbre Parameter Page) but you can't monitor it.. Read on..
The easiest way is to bring the combi into the internal sequencer and record it out to your DAW from of there.
This is my work-around:
Lately, when I don't want to use the I.S. what I have been doing in combi mode is "duplicating" the drum track (Program and channel.)
You set one of them to EXT. The notes for the drum channel will now be recorded and you can monitor them because you have a duplicate still set to Internal.
When setting a channel to EXT in combi mode, you can't monitor it. That's why I set the duplicate..
If I'm missing something or someone has a better way, I'm all ears.