As far as I understand what you want to do is connect people who already exist on your tree, in a different way.
To do this you can use the attach detach tool in FTB.
So Attach, as opposed to Add, lets you draw a connecting line between two people who are already in the tree, and Detach removes that connection but without deleting any of the individuals. For each type of relationship there is a way to Attach or Detach. For husband and wife this feature is located in the "Spouses" menu. For children it is located in the "Children" menu. For parents it is located in the "Parents" menu.
There are multiple ways to reach these menus in order to invoke the Attach or Detach functionality. They are all the same so you can pick the one you find most convenient.
A) On the main menu, under Person -
You can reach the Spouses, Children or Parents menus.
B) By clicking on the EDIT button on an individual's genealogy card
C) By clicking on the EDIT button in the "Family" section.
My recommendation for using the Attach/Detach feature is to access it through the MANAGE function (e.g. Manage Spouses or Manage Children). You can always do an Attach/Detach without using this screen, but in the MANAGE screen you can see and adjust ALL details at once, which is very helpful.