We don't really merge trees on WikiTree so much as connect our immediate family to the larger tree. Once you have built your tree to the point where you can connect someone in your tree to a profile already on WikiTree, usually a parent, you're all set.
Just go to the edit page of the profile you want to connect and click on the relationship you want to add, mother, father, or possibly a spouse. Enter the Wiki ID of the profile you're connecting to and click "Go."
Here's a very helpful link: