Pokémon Diamond and Pearl/Dual-slot mode

Dual-slot mode, is a way that you can use to obtain Pokémon that are out of the Sinnoh Dex, by putting the GBA Pokemon games in the DS slot.

With any of the five Game Boy Advance games inserted while playing either Diamond or Pearl, you will find wild Pokémon not otherwise available in Sinnoh (except migration at Pal Park).