You cannot enter DNA information for another living person for privacy reasons. If your uncle is living you can create a profile for him and add his e-mail address; that will generate an invitation to join Wikitree. Once he joins and enters his DNA test information then he can turn management of his profile over to you if he isn't interested in participating.
If he is deceased and you manage his profile you can add his DNA test information from the "Family Tree and Tools" menu on his profile.
Here is a link to the DNA Help pages: Category: DNA Help (wikitree.com)