2 Ways to Rome.

Regroup externally, and use the fitting room to transfer rig and morphs.

Do not regroup, and open the Joint editor.

Select the left thigh of the dress => Goto the Joint editor and go to "Bend" and in the right upper corner you will see a box called : "add element to deform". There navigate and add the left shin. Now you can repaint the weightmap for the thigh and if required also repaint the bulge maps.

You will have to do this for Bend, Side-Side and Twist to get the dress following completely.

Best regards, Tony