I had to do that, to a hat someone made for me - too short - I just slipped the end stitches over the loom (round) that fit best, and continued adding several rows. I had to use a contrasting color, as I didn't have their yarn to match -
Follow a you tube vid on how to add a rim or brim- It was surprisingly easy to pretty much re-slip the "finished" stitches on the bottom, back on a loom, and then e-wrap as usual and continue knitting a new length, and then finishing it off again -
I wish there was a way to post photos here - Don't be afraid! Just try it!