GBA Sprite Editor?

The only way currently to edit GBA sprites is to use something like YY-CHR and draw the tiles one by one with a reference sprite sheet. As an example, the character sprites start at $760000 in the (U) version. You could try loading a zsnes savestate in YY-CHR to get the correct palette and if the color order is the same on GBA, the palette would be accurate.

You could also try to convert a spritesheet to GBA 4bpp with this Gimp plugin or another utility and use two instances of YY-CHR, one with the converted spritesheet and one with the GBA ROM and copy/paste the tiles from one instance to the other.