Creating an instrument from samples from multiple presets in instrument.xml?

Hello,
Is it possible to create a percussion instrument in a custom instrument.xml from different preset samples in Musescore's default soundfont, or do I have to create a new preset within the soundfont? For example, let's say I want to define an instrument in an instrument.xml file with samples from the two different soundfont presets: program 0 (Standard kit) and program 96 (Marching tenor drum). (I am guessing that the elements "96" refer to a preset defined in the soundfont.). I don't see how unless one can refer to more than one program under channel. Is my understanding correct that the instrument.xml can access only defined presets in the soundfont, not individual instruments or samples that make up those presets?

Thanks,
Sam