When I was telling you about cutting sweaters and poking holes in them and then crocheting around them, I was talking about manufactured sweaters—not hand knitted sweaters. I have found with most manufactured sweaters (those that are machine woven), you can actually just cut them and crochet right on the edges without them fraying much (you might find a few tufts of fiber come out, but after a few washings, they stop). I tried zigzagging the edges, and it only stretches the fabric and makes the edges all wavy.
I cut the front out of one sweater (I poked-a-hole-in-it and crocheted all around the opening). Then, I used the sweater pieces for patterns and laid them down on the crocheted motifs and cut the crocheted fabric; then, I took the crocheted pieces to the sewing machine and zigzagged all around them. Then, I crocheted around the crocheted pieces (to make them straight). And finally, I crocheted the two pieces together. That was about 5 years ago, and that design is still holding up after many washes. I have also cut the cuffs and bands off of quite a few sweaters and added edgings to the bottom of the sleeves and the edges of the waists, and all of them are still holding up.
This is a picture of one of the sweaters that I added lots of rows of different colors and edging to in order to make the sleeves longer. I like it because edging can jazz the sweaters up just a little or a lot depending on how many rows of edging you add. That’s what is so fun about adding crochet to sweaters.