This is basically just expanding on what IceCold said, but here it is anyway...
What I believe is that in order to accept data on the GBA cart a DS game needs to do these things:
-Read the GBA cart (Obvious, I know)
-Verify that the cart is a compatible update (My copy of The Minish Cap shouldn't be patched to a game)
-Patch the information the game is looking for into the game (Such as added music)
What I'm trying to say here is that game is probably looking for certain things (new songs, new race tracks, new characters, etc.). This means that it doesn't just take the entire GBA cart and apply it as a patch to the DS game without discretion, but instead only takes what it can verify. So, unless someone can disguise online functionality as a new melody or find a game that is programmed to look for it, I don't think you can easily add things like online functionality to the game.
In short, the game will only use expansions that have the new "stuff" the game is looking for.