Hi teke,
In general, the answer is no for your second question. If not rated for -40, don't install in -40.
There are exeptions.
The general requirement that the equipment be listed is the limiting factor in my opinion. By installing in an environment below what the manufacturer says, the listing is violated.
What sort of equipment is it, what voltage(s) hook to it, and where in the world are you? Tell us those and you might get more definitive answers.
Good on ya,
Goober Dave