When you save at Pal Park you have a choice to migrate pokemon from your GBA games. It allows you to select up to six pokemon from your game. You return back to your game, and then you can enter Pal Park to capture your pokemon. Once you've caught your six pokemon, you can choose between having to recapturing them, or puttung them in the PC. If you put them in the PC you get to keep all six of them.

WARNING: Using Pal Park to capture pokemon from GBA games will not allow you to send them back after they have migrated, be cautious about it.