Sure.
Even tough in the FAQ is mentioned "We ask users not to enter giveaways for packages if they already own the majority of the games".
But I guess in the end it is up to you.

FYI this is a single key for both the main game and the DLC.
If you are lucky enough and win, and since you already own the main game, you will only get the DLC from the steam key.

Cheers.