I'll explain in more detail, basically if you want a Mew, Celebi, Jirachi or Shaymin etc. Bulbapedia says the only way you can get them is by a global event or through trade.
So what I want to know is can you see them in a Pokemon game such as Black/ White 2 to add them to your Pokedex.
The reason I'm asking is because if you go on the GTS you can request the Pokemon your looking for if you've offered a Pokemon and on GTS Negotiations you can edit your trainer card to put the Pokemon your after.
However you can only do this if you have seen or caught the Pokemon your looking for before.
Therefore I would like to know if there are any possible ways you can see an event Pokemon in your game. This could be through an in game battle (because legendaries have been used by other people, as I recall Alder's son using a latios), an online battle if it updates your Pokedex or by any other means necessary.
Because if I can see an event Pokemon to update my Pokedex it would be a lot easier to get one through trade.