OK, i'll PM you, and then, i'll re-post a mini guide XD
edit: OK, so
1. Start the GBA game you want to dump the save
2. Start Decrypt9 and dump the save (SysNAND Option ->Miscellaneous -> GBA VC Save Dump )
3. Edit the save
4. Start the GBA game you want to inject the save (only if you have turned off the 3DS before),
and start decrypt9 WITHOUT turn off your 3DS (you can reboot the 3ds using system settings).
5. Inject the save (SysNAND Option ->Miscellaneous -> GBA VC Save Inject )
6. Start the GBA game.
7. Profit!
Update: If you have trouble with save files that are 64 KB instead of 32 KB (for the injection), try this:
Here an example with a save of fire emblem sacred stone.
Open a hexadecimal editor (like HxD), and erase all the null content at the end (the save file have to end by FF, and have to start with "AGB")
So, like on the images, erase all the blue content!
Now, the save file should be 32 KB.
So, the save file should end at the offset 00007FFF.
Note that the save cans change depending of the game. So, i recommend you to dump the gba game save, and compare it with the save you have downloaded on the Internet, to remove the right things.