Unless backed by fabric, I would think that a knitted or crocheted edging longer than 1/2" to 1" would tend to look limp or hang funny. As the bottom of pants, it's also likely to catch on things and get snagged if it's at all long.
If you add fabric to lengthen the pant, it's very important that you keep the same taper as the leg or the added fabric will pull or hang oddly. The exception to the same taper would be if someone were making boot leg or bell bottoms.
If you are after fun, funky and unique, it could be worth the effort. Otherwise, a new pair of pants or a pair from the thriftstore that are the length you want might be a wiser use of time and materials.