Electrical: How do I connect copper ground wire to metal wire passing through switch box?
Each of the 'conduits' as you say, have an uninsulated ground conductor.  If there are three such 'conduits' then there has to be a splice joining them.  Add your ground there.

I've installed many insteon devices before.  The first thing I do, and my best advise, is to spread out all the wiring after removing he old switch and before installing the new, to know absolutely what you have to work with.

If you absolutely have to connect to a ground without cutting and splicing the original, I would recommend a split bolt. 

It's going to look weird as hell in an outlet box, but if you thoroughly clean/abrade the existing ground so it makes good contact, a split bolt should do the trick