To be honest, I’ve never tried sewing on pantyhose before so I don’t know how difficult this would be, especially since the size of pantyhose changes whether or not it’s on your leg. If it was me, I’d practice on a cheap pair to see if you can even do it. I’d also feel more comfortable stitching it on a serger rather than a sewing machine as it seems like it’ll hold up better and possibly still provide some stretch, especially if you use wooly nylon thread. I’d put on the pantyhose, pull the excess to the top of the leg so you can pinch and see how much you need to remove. I don’t know if you can risk putting straight pins in at this point or wait until it’s off your leg so you don’t accidentally get a run. But because it’s going to be different once it’s off your leg, I don’t know if it’s really going to make a difference and you might have to stretch it as you sew. But, truthfully, this is one alteration I think would be really difficult to pull off and I don’t have any good answers for you. Sorry I can’t be more helpful.