At present the way to share problems is to send them to Mike or me. How to do so also answers your "canonical" path question. The path we suggest you use when using the "Save as" feature is the path that is presented to you
(e.g. something like
and then change the name of the problem. The above problem was named ur (for Rochester), dr (for derivative) 5 (for subsection 5) and 18 (for the 18th such problem). You can use your own scheme but this one works. Of course this assumed your new edited problem is on the same topic as the original one. This way at the end of the semester all your new problems are in the "local" subdirectory of the course's templates directory and if you tar up that directory and send it to us, we know where to put the problems in the National Problem Library (NPL).
The other requirement we have is that problems must be tagged to go into the NPL. You should just be able to modify the tags in the original problem (see http://webwork.maa.org/wiki/Tagging_Problems).