The link from CarmenIbanez is a good one, but I don't think you have to get that fancy.
Yes, you can insert a lifeline -- as a matter of fact, it's a good idea. There's a video about that under the Basic Techniques / More section of this site. It's under "Fixing Mistakes," and it's called "Inserting a needle into a destination row - before unravelling."
What I was trying to convey earlier is that you have to pull the entire thread through every stitch when you're unravelling from the bottom. So cut a stitch or two right below your destination row, then pull out the yarn from each stitch. You can cut it every once in a while so you don't end up pulling three or four feet of unravelled yarn through every stitch.
Then you'll be left with the portion below where you cut off. You can certainly frog that from your cutoff point -- remember, at that point, you're unravelling downward, and it acts like any frogging. If you don't need the yarn, you can just throw it away without frogging, or use it for a headband, or something. But you can certainly frog it and use it in your reknitting.
Does that make any sense? I think that once you take the plunge and make that cut, you'll see what I mean about pulling the yarn through the stitches, and you'll figure out how to proceed. It's hard to describe, but easy enough to do.