They possibly could... and if the DS's slot-2 were capable of reading GB/GBC game paks, they even could've allowed the DS games to migrate gameboy and gameboy color Pokemon, reglardless of whether the GB/C and GBA games could link or not. However, the Pokemon you would get through migrating wouldn't really be the same... Generation 3 introduced a completely new (and more complex) system for a Pokemon's stats, and included things like nature, etc; and this was carried through to Generation 4. To send a Generation 1 or 2 Pokemon to one of the DS games, all that additional data would need to be generated for the migrated Pokemon, similar to how a wild Pokemon's stats are randomly generated upon encountering them. For that reason, the only things about the Pokemon you'd end up with on the DS game would be the OT name, ID#, nickname and (most likely, assuming the migrate feature included code to detect and accomodate for) shininess.