Question about patch stud wall (large one)

Hi everyone,

Please see the attached video. I’m going to be creating a part of a stud wall to patch up an old cupboard so that I can give the space to the next room over.

What I’m not sure about is if I can use the existing frame that’s already in the wall, or if I should make my own (the four sides of the stud frame).

It would be great to hear how you’d approach this. Many thanks.