Can I add an existing person in the family tree as a son/daughter to another existing member in the tree

Answer 1

1. Click on the son or daughter
2. Click the "Change or delete" button
3. Click "Change parents"
4. Use the first two drop down boxes to pick the people who are already in the tree to be the parents
5. Use the third dorpdown box to set what kind of parents they are.

This functionality is one of the most hidden features in the user interface. i wouldn't feel bad about not being able to find it. My suggestion was to make a parent's tab, like the Partners tab and put all of the parent related functionality there.