There is a way. Go to the following help page:
http://www.wikitree.com/wiki/Adding_Family
You'll find a link near the top of the page that says, Add an unrelated person here.
The information on the page includes:
If this is a family member it's better to add them as a parent, sibling, spouse, or child from your profile or from someone else in your family tree or Watchlist instead of using this form. That connects them to your tree.