Yes you can
There are a couple ways to do it but all of them start with removing the current wefts from the wig cap. Pretty easy to do, just get a seam ripper and start breaking the seams on the inside that you know are holding the wefts down.
Once the wefts are removed you can put new hair on in one of two ways: make wefts yourself or "ventilate" the cap.
An easy way to make wefts that I saw someone on DoA do is take some similar colored yarn and cut it into lengths for wefts. Take your hair strands and pull out about 5 to 10. Fold in half as if you were going to root the hair, but instead what you are going to do is knot it around the thread as if you were doing a latchook rug. I don't have a better way to explain that I'm afraid so you may have to look it up. Just keep doing that along the yarn until you have a weft the length you want to sew back on. Repeat until you've enough to make your wig
There are other weft making methods out there - if you google you can find them, but I always thought this one sounded easy to me.
Ventilating is the technical term for putting hair directly on a wig-cap - it's the method used by professional wig makers and it's not hard, just time consuming and takes a bit to get the hang of. Remember how I mentioned knotting hair onto yarn as if you were doing a latch hook? Well that's basically what ventilating is only you are going to knot the hair directly onto the wig cap!
Or, third option is to buy wefts that are premade
