>Can you add premade single forms to a formset?

I don't think so, no. Not at design time.

As far as I can remember, it's a special record in the .SCX that determines if it's a formset or just an ordinary form.
Then the forms are created in the one .SCX file and you can't add them, only create new ones.

I can't come up with a good reason to use formsets.