I know that there are customizable characters in SP, so are you going to be able to use that character in MP too?
If you weren't able to, it would be kind of stupid...
The same model and skin? Sure.
The same stats and powers? Unlikely.
I doubt if there will be an RPG-style game mode where you can put your character into MP and play with their powers. A mod would be cool for that, though. Problem is, if you get stuck in SP, other MP players will start to be a lot better than you right away...