I was assuming he was asking if there was a way to load a homebrew program on his slot-1 that would act as an Action Replay for an official GBA cartridge inserted into slot-2, and that is not possible. The opposite is possible, running the DS homebrew program "NitroHax" to apply cheat files to an official DS cartridge (you eject the slot-1 cartridge and insert the DS game)
In addition to the site shadowman posted earlier, there's a site on PocketHeaven with lots of GBA game trainers as well. Google search to find it, and apply the IPS patch files with any standard IPS patcher (to a COPY of the rom file).