In all honesty, don't add any more. Let me explain why:
In GBA hardware, there is a max of 256 OAM colors, (this is 16 OW palettes) on screen at once. Furthur limitations in the coding of the game only allow palette slots 0->A to contain pallets for OWs (the others are reserved for other purposes). There is a reason GF coded the games to use so few palettes: only 11 unique ones can be on screen at once! I hate how JPAN added new palettes, because it drives me nuts that you really can't use them. I'm all up for adding new sprites, but the number of palettes can't change much, otherwise you will have a very limited number of characters on screen, and you will have to be very careful about where you place them, otherwise you will get a lot of OWs with the wrong palettes, because a new one was loaded that used the same slot.
Furthermore, each OW has a fixed palette slot. This means that it has only one slot which its palette can be loaded into. Do you see the dilema? If you have too many palettes, (more than 11 actually) these slots have to be shared. So, if two OWs share a palette slot, that means that only one can be on screen at a time, lest the second one to be loaded will overwrite the first one's palette, and the first will look like crap.
So, long story short, don't add new palettes.