Add own instrument

You can edit the instruments.xml file. It's rather sparse, but here's the documentation for it. https://musescore.org/en/developers-handbook/references/instruments.xml…

We should make a distinction between "instrument" and "sound" here. Adding you own instrument is easy. Simply add any other instrument with a similar sound, then right click the staff, Staff Properties, and change the name and whatever other properties you might need (clef, transposition, range, etc). If you plan to use this instrument a lot in similar ensembles, you can save a score to your Templates folder. If you plan to use it in *different* sorts of ensembles, then you can create a custom instruments.xml file as mentioned above.

However, all of these will still use the *sound* of the instrument you started with. To add a new 8sound*, you need to add a soundfont - see the Handbook under "Soundfont" for more info. Possibly there already exists a soundfont with samples of the isntrument you want. If not, however, you will need need to create your own soundfont, and that's a lot more effort than anything else described here.