Move my partner to my plan

My partner and I both have AT&T plans and both have iPhones. Hers is a legacy unlimited plan. I would like to move her existing AT&T number in a way that I pay for both lines, switch to a family plan, and she keeps her current phone number. How do I do this?

I just got her an Apple watch for her birthday which is not compatible with her plan. I would like the new plan I get for both of us to include that