If they would officially have that feature then they would need to support it. There is a lot of potential user error involved when adding custom fonts. Put too many, your font selection window cuts off the bottom buttons. Put fonts with wrong metadata and they might not be linked to same family. Be happy that you can add custom fonts at all - without even having to jailbreak. As for the hidden fonts - why see them in first place, they are for exotic languages like Chinese and Japanese. That they are included in a stock US PW is pretty nice of Amazon.
Plus, what ixtab said.