Eh, When I say a waay too much work, I mean you would have to:
-Go into and add the pointers that are needed to identify a pokémon. I have no idea what is specifically required.
-Repoint arm9 so that it doesn't crash immeadiately, which would probably take days.
-arm7 might need the editing job, too.
-Multiple scripts and text need to be edited to include the new pokémon
-Script functions would need to be redefined probably
-You would have to sprite, animate, and give this pokémon a new cry, stats, movesets, etc...
o_e
See what I mean?
This might not be everything that needs to happen and It may not even work.
It best to just edit over what's already there at the time being.
BUT Black and White 2 DO have those Props for PokéStar Studios "Pokéwood" that aren't related to the main game and have all the things required of a pokemon EXCEPT a dex entry.
Catching these props cause the game to crash, as there is no pokédex data to display.
If we could sidestep this somehow, We could introduce at least 20 pokemon into the game without erasing any existing ones.
I'm actually working on this at the moment and will take advantage of this in this project.