Cutting/extending/patching zippers?

Was just reading the thread on the melted zipper (and I can just imagine how frustrating that was for the poster!) and was wondering about the posts that mentioned patching in another piece of zipper, or extending a zipper.  How does that work?  I've only ever sewed in zippers that were already the length I needed, but would love to be able to adapt the zipper length to specific needs, especially since much of my stash is actually thrift-store clothing finds that I cut up, which can be an abundant source of zippers, although not necessarily exactly the size one needs for a particular project.

I don't know of any way you can lengthen or patch a zipper, although you can cut them to the correct length.  You can also replace broken sliders on most zippers.  I work for a dry cleaner, and do A LOT of zipper repair, but if the zipper is missing teeth or that little part that inserts into the slider on a jacket zipper, it's pretty much a done deal as far as I know.