>I have a formset containing one form. How can I add an existing form to the formset?

You have to create a Form Class to drop/add it on the formset.

1. Create a class of the form (if you don't have one, use Save As Class' from
file menu pad.)
2. Add the VCX containing the Form class in Form Controls Toolbar.
3. Select the form and Drop it onto the formset same as you drop other objects.