Customizing Multiple Instrument Channels
MuseScore's got a handy feature that allows you to switch, for example, between tremolo, pizzicato, and arco for stringed instruments: 
But the multiple instrument voice channels only seem to appear with certain instruments, such as stringed and brass instruments, and some synthesizers, and you can't add, substitute, or delete voices. I was wondering if there was any way to customize the instrument voice list? It would save room on the page to be able to put multiple instrument voices into one part instead of having multiple parts, especially when in real life, you can program a synthesizer to play just about anything. 
Comments
Currently the only way is to use a customized instruments.xml for the channels and no way (as far as I know) for these MIDI actions
And since, as you say, a synthesizer can change to anything, customziing the instruments/xml to make all of these channels available all the time would be a mistake. Instead, simply use the Instrument Change element in the Text palette to change to any instrument you want at any time.
In reply to And since, as you say, a by Marc Sabatella
But note that you won't be able to use it to change back to a channel you already used: #42301: Instrument Changes: add ability to change back to a previous instrument