I found out hex locations for EXP.
00000028
0000006C
000000B0
im not sure which location is for which character. Also, I would avoid adding too much exp at a time to avoid going over the next rank. Im not sure what would happen if edit your exp past the next rank bonus if you will get to select a bonus or not.
EDIT: I just checked and if you put more exp than the next rank up, you DO NOT get to choose a rank up bonus. Here are some numbers to get you close to the next rank. You will have to level up from fighting a enemy to get the rank up bonus.
Leaf - 39500 EXP
Star - 97500 EXP