Of course, but there's no guarantee they will be accepted. They will need to be of excellent quality and able to be released under the GPL. Also, there needs to be a balanced array of guns, and what we have now works pretty well, so we won't be adding any guns, but instead replacing old ones. It would be solely a visual change.
Also, even if they are accepted, it could be a while before they're put in the game. New weapons are pretty low on the priority list right now.