Unless you Give them to Hold, then trade the Pokemon to the game, then there is only one other option.
It's called Gameshark!
Nintendo don't make them for GBA, so you might have to buy one from EBay or something.
If your not playing on a GBA, but a VBA (VisualBoyAdvance, which is basically an Emulator you use to play GBA games on you computer), (MAKE SURE THAT YOU DON'T HAVE ANYTHING UNRETREIVABLE IN YOUR FIRST SPOT ON YOUR PC, BECAUSE YOU WON'T BE ABLE TO GET IT BACK), then you can simply click "cheat" up on the tool bar, and then hit "cheats list".
A small window will pop up. Click "Gameshark" and Type any thing in the top text box. Then in the Bigger text box, Type: 0000B138000A 1003A82A0007
This is the MASTERCODE. This must be on for this to work.
Click ok, and click Gameshark again
Then type "rare candy" or something like that in the small text box, and type: 82025BCC 0044 in the big box.
The go to the nearest pc, and look at your items. The Rare Candies should be there.
Hope this helped.