I don't know exactly how to edit the font, but i know how to import it.
The FE Roms has two Pointer Arrays that have pointers for each symbol, one for the text font and other for the menu font, these poiinters points to a 0x48 bytes of lenght block that are the symbol info, you need to import these blocks for the symbols that you want to add and point them to the new free space...
But this is only neccesary on the FE7 USA rom, the FE8 USA rom has the Spanish-Francais-Portuguese-Italian symbols in it
FE7 USA Menu Font Array - B896B0
FE7 USA Text Font Array - B8B5B0
FE7 EU(English-Italian-Spanish ver.) Menu Font Array - C4C90C
FE7 EU(English-Italian-Spanish ver.) Text Font Array - I think this one is in C4F704 but im not sure
they start with 31 empty slots because they follow the ASCII code so, this is the order
In example, if you want the "Ñ", you need go to the FE7 EU array, go to the slot 0xA5, follow the pointer, copy the 0x48 bytes block, put in a new space in the new rom, go to the 0xA5 slot of the FE7 USA array, an put the pointer of the 0x48 bytes block
PS: If you speak spanish i can explain more easily