Yes, it can be done. There are better advisors than me though.
Maybe Suzeeq or another Mod will weigh in on the subject with links and so forth.
But, here is what I did to lengthen a poncho for myself.
I picked up stitches along the bottom edge, and knit down from there. Of course, everything was st st, not ribbing.
Is there a possibility that you could simply add to the ribbing for the extra length?
If not, you could take out the ribbing, pick up the live stitches, and knit down from there in pattern, then finish off with ribbing.
Some would recommend running a safety line along/through/into the st st row just above the ribbing before ripping out/tearing down the existing ribbing.
Anyone else care to add to this? I'm sure there are other ideas!