Use hourglass9 to inject save data.
- Load the pokemon game. Then close it and return to home menu. (THIS IS IMPORTANT)
- Open options and close it.
- When you close options, hold assigned button to load hourglass9
- Use the option to inject GBA save data, make sure the save data file is dumped from the GBA cart you own and is in the files9 directory on sd card.
You should check it with a emulator first on pc, i use visual boy advance for it. (Note not every game will work with VC injection, and if game has issues most likely will save data, most of the problem comes with save data conversion to VC compatible format from original intended that various GBA games use and VC emulation not being updated to support more games to begin with.)