Adding exta rows to finished project?

Hello. I have a project that calls for a 2 in border of k1,p1rib on both ends of two rectangular pieces that will join together horizontally. Unfortunately, I am a bit short from my casting on side and I would hate to unravel the whole piece. Can I add a few rows to that end or do I need to start over? Thanks.

Do you mean that you did less than 2" of ribbing on the cast-on side? You can unravel the cast-on edge, pick up the live stitches, and add more ribbing that way. If you leave the cast-on edge and just pick up new stitches to add your ribbing, there will be a very obvious seam there.

Hi Abbily
I just realised that I ribbed 30 rows instead of the required 36 on my completed back of a long cardigan back.
So if I unpick the cast on edge and pick up stitches like I would a cast off row I can then rib the extra rows and then cast off and it will be the same?

It’s not so easy to undo the cast on but if you’re patient, you can do it. It may help to use a smaller needle to pick up sts a row or two above the cast on first. Then you just need to unpick and not worry about grabbing live sts as you go.
The problem is that the sts will be off by half a stitch, not a problem with stockinette but it is with ribbing. It might be easier to cut the knit fabric at the join between ribbing and sweater body (possibly this is stockinette?), pick up sts and then knit the ribbing down for 36rows.