You could use an external mixer, but you could still only input from the mixer to the br900 the amount of mic inputs that the b 900 has. What i mean is that you'll probably end up with the whole drum set on a stereo track (left and right) and not each element on a different track (if you record it live - all at once).
There's not much choice. You can record everything with a couple of overhead mics (but you couldn't edit individual elements after that). or use the mixer but it would come down to the same thing.
If you want to be able to edit and isolate each element you'd need an interface or device with the correct number of inputs.