Yes. Click on the person you want to add another set of parents for, you'll have the option to create another set (foster or adopted) or set their secondary parents to someone already on the family tree. You also have the ability to click on the main person, then click on the button "Change or Delete". At the top should be "Change Parents" and you'll have that ability as well as the ability to assign a set to whether they're foster, adoptive, or biological parents.
