Just click in the box and it'll give you a list of other options. You can add your own, but it doesn't calculate the amounts. Just noticed these myself, was thinking of using maple syrup for some bottling I'm doing today. That may be handy.
Edit: actually playing with that it looks like it only automatically populates white cane sugar. I'll have to go elsewhere for the maple syrup.
Edit again: just worked out the numbers and that looks like a waste of time. I think I'd add in secondary rather than use it as priming sugar if I wanted that flavour.