I suspect the database (library) comes seeded with a few standard ones but then this list gets added to by the ones from the imported/edited media files. I doubt it's available as an editable list.
If you edit some (dummy?) files with the genres you're after you should be able to see them and select them from the tag editor list.
Setting up a 'Panes' view with 'Genre' in the Show categories (along with album, artist etc etc) will give you a quick overview off all the genres in use and what's been tagged with them. That may help do what you're after (at least the editing/view bit)
Spike
