There is currently no way to include an individual who is not related to another person on the family tree. I'm not sure just what you're trying to do, but you could possibly do what you want by adding an intermediate "stand in" for the part you don't know. For instance, If you know that they are a descendent of Martha Washington, but you don't know how, you could ad a child of Martha Washington with a name like "Unknown number of generations". Then make the person a child of that stand in.
You could do something similar in the opposite direction if you know an ancestor, but not exactly how they are related.