Key and Contribute

no you can't.
keys you use are tied to your account

If you've already used the key, that's it, it can't be used again. So no, you can't contribute it.

No, you can't give away games from your library. And have a look at this

You cannot give away a key that has already been used and you cannot give away games you already have activated on your account.

Okey Thanks all :D