Wiring – This Thin Metal Wire

The correct answer is: look up the code in your area. In some parts of the USA you can't get insurance on your house unless a licenced electrician does the work -- quite possibly with a construction permit!

If there is no code, then as a minimum I would:

1) get a hammer drill and make a proper sub-ground hole in the foundation large enough for a conduit (personally I'd use 3/4 PVC or similar).

0) Dig your trench and expose the foundation where you want to route the wiring.

2) install the conduit and pull the wiring - make sure the gauge is sufficient for your end needs.

3) seal the conduit where it goes thru the foundation using hydraulic cement.

(yes, you OCD-ers, I know I wrote them in the wrong order)

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