I need help adding another instrument in my project that doesn't exist in the .xml

I don't know if somebody else has written about it but I want to create an instrument (Greek Bouzouki to be precise) to write my sheet but I don't really know how stuff works? Could you help me?

P.S. Sorry for my english. It's not my mother tongue.

Edit: Also, I don't remember if Bouzouki is in the instrument list. I have many weeks to open the program. :-)