If you're knitting it in stockinette it WILL curl. It's the nature of the beast. In order to prevent that you need a non curling border like garter or seed stitch. If you want to fix it after the fact you'll have to pick up all the stitches and then do a border. Not sure it will work as well that way. Best thing to do is a swatch and see how it works for you either way.
When asking questions ALWAYS post the name and a link for the pattern if you have it.