(((((( I'd have to see the item to be sure, but I have a couple options for you:
Knit edges can be applied, too after the project is completed if you want to stay true to the craft form. See:
This might get tricky, though as you'd have to pick up stitches on the edge with needles where a crochet hook can be applied anywhere on the item.
I'd probably NOT put a regular single crochet stitch or detailed crocheted edges of more rounds on a knit item but would consider using this crochet stitch:
RSC (reverse single crochet).
This stitch is aka crab stitch, etc. It's literally a SC
going the opposite way, produces a stabalizing final
round (finishing stitch) and at the same times makes a
CORD look which is lovely no matter what the material.
I'd experiment with the hook size suggested on the yarn label, pick up in a corner with at least two RSC, perhaps three but it depends on your edge design of your knit afghan. I'd work loose to start and see if it needs to be tighter, drop down a hook size or work tighter with the first one.
You'd have to have enough yarn, too.
I can help you more if you'd send in a pic of one corner and the entire edge of your knit afghan if you have it there, even if it's in process.
Even for an older afghan that needs a stabalized edge, one can apply this RCS after the fact. Perhaps using a contrasting color yarn in a similar weight would look nice, too.
I have a question. Some of you that crochet can answer. When you knit afghans they are not as stable as crocheted ones (don't hold their shape as well). My question is should you crochet (sc) around the edge ?