Is it possible to add a new instrument? What are the necessary steps to
do so?
Hey, Tobias. I think those are just names that map to General Midi [1] instruments. On the back-end it is just assigning a number to identify which instrument you choose so that in a software or hardware using the General Midi specification you would _more or less_ know which instrument you will hear. I say more or less because even if instrument #1 is "Acoustic Grand Piano" depending on your midi device and "Acoustic Grand Piano" will sound waaay different depending on the sample sounds. MIDI is just a pianola roll, the actual sound you will hear depends on the pianola.
That said, what I would do if I was you is just to choose a percussion instrument (e.g. "taiko drum") and then using a midi software to map this to the zabumba sound you want, provided you have that sample.