You can trade post National Dex (but you have to reach a certain town, to my knowledge, it's pretty early in the game so I assume your somewhere in the middle of the game)
However, to get your Treecko, you must have your White 2, you must have another Generation V Pokemon Game that has access to the PokeTransfer (i.e. It's a completed game), and you must have another game (preferably Gen III) that has Treecko on it.
- Get your Nintendo DS, insert your Gen III game into the GBA slot, and your Gen V game in the DS Slot. If you are using a Gen IV game, instead of a Gen III game, you'll need two Nintendo DS systems and you need to use Nintendo DS Download Play.
- Head to the PokeTransfer and migrate the Treecko to your completed game. Make sure it does not have any items, since items cannot be transferred.
- The Treecko should now be in your completed Gen V game. Now, get your White 2 and trade the Treecko over to White 2.
Treecko should now be in White 2! Congratulations!
When you are summarizing Treecko, you will see that the Pokedex No. simply reads ---. That is okay - when you get the National Dex it will read #253.
Hope I helped. :)
(Sorry if I overdid this. xD)