Custom RB2 gens sZGx and sZHx not supported

[r0bd0g]

This is an inconvenience I've ran into a few times now. My guess is that these are arbitrarily being prevented from working because they're not official gens. I would suggest a way to bypass that check and convert the WAD anyway. This could make sharing of a couple of custom songs easier since people could use processes they're already familiar with, rather than figuring out how to pack the songs for real. (But since the number of slots is limited and everybody would use different slots for different songs, it can't really be used on a wider scale.)