Is this hand knitting or machine?
Are you planning on adding the extra at the shoulder end or the hand end, sorry I was unsure from your wording.
If I was hand knitting I'd cut a narrow strip off the cast off edge if I couldn't unravel, and then unravel neatly as fr as I wanted to go, and then start knitting again.
But that may be of no use LOL
Good luck with it.