1. get CrystalTile2 & a hex editor.
2. open the rom in CrystalTile2, select (V) in the top menu bar, and switch to Hex Editor mode if it isn't already there.
3. at the top menu bar, select (T), then NDS File System. a small window will pop up.
4. right click "Arm9", select (U).
5. using a hex editor, open up the newly extracted & uncompressed Arm9 file.
6. search the file for 98 00 00 00 9B 00 00 00 9E 00 00 00
7. change the 98 00, 9B 00 or 9E 00 to the hexadecimal value of the Pokemon you desire, and save.
8. in the same pop-up NDS File System window in CrystalTile2, right click Arm9 again, and select ©. now select the edited Arm9.bin.
9. close all programs. an alert will pop up. select the option that isn't cancel.