It is possible to take out a cast on, but it's not as easy as taking out a bind off where you can just undo the last stitch and rip it back. With a cast on you will have to pick each stitch apart one at a time. There are a couple of options though. I've done this several times and it works great. Insert your needle a few rows up from the edge (see Amy's video under fixing mistakes called 'Inserting a needle into destination row, before unraveling'). By inserting the needle before ripping, you ensure that you don't lose any stitches. Now, take a scissors and cut the cast on edge off, just making sure that you don't cut anywhere near the stitches that you've just put your needle. Once the cast on edge is gone it should be relatively easy to pull out the little bits and pieces from the cut and find an end that you can just pull back and unravel to the stitches that are on your needle. From there you can just knit as much ribbing as you want and then bind off loosely.
The other thing you can do is just leave the piece as it is and rather than taking out the cast on, you can pick up stitches along the bottom edge and add ribbing that way.