Yes'm, you can undo the cast-on edge or just pick up stitches.
If you undo the cast-on, be aware that your new stitches will be moved half a stitch over. This is hard to explain, but you will see why when you do it. It doesn't really show in stockinette, but does show in ribbing. A lot!
Here is an example of how I dealt with it. See the vented part at the bottom of this sweater? I added it on after the fact. I did one row of garter so it would be harder to tell that the ribs were 1/2 st off.
Of course, if this is a part of the hat that will have a brim folded over it, no need to do anything!
Undoing the cast-on edge is a PITA, if I remember right. Bear with me for a second, you can actually CUT it off with scissors, and then just pick up the second row of stitches that are now live. I promise