Not having much experience with other design suites, I can attest that you can import and edit STL's in Sketchup, with the right plugin. They do tend to import a LOT of extra geometry that makes it a pain to edit though.

I would do a google search on importing STL into your preferred editor. Other packages may handle them better than SU does.

Another option I do on occasion is to import the STL, then manually create an exact copy overlaying the original. While this can be tedious at times, it makes for cleaner geometry and therefore easier to edit.