Hi, Tiggs! Good question.
I thought I knew what to do, but my idea didn't pan out either.
Over how many rows do you need to do this?
I had thought you could use a circular needle and slide the stitches to the other end (slipping the stitches back to the left hand needle works too) and then start with another ball of the same yarn back on the right side. That works, but you still end up with both the yarns at the wrong end. I was hoping I would have one at each end, but not so. If the lady in the store only showed you one row, maybe that is what she did, but it won't work repeatedly. Well, it will, but you will have multiple ends (You could just as well cut the yarn after each row and do the next row with the same ball) to work in. If you don't have too far to go that might be a workable solution, but if you have a lot of rows you would have a lot of ends.
Just an idea and maybe it won't work for you at all for some reason, but knitting in the round you will always be working from the right to left, could you do the sleeve in the round to accomplish this? You would most likely need double pointed needles or two circulars or do a Magic Loop with only one long circular, even small circulars are usually too short to do a cuff.