Can I use epoxy resin to fix a metal hook to a uPVC door, to stop it blowing in the wind?
Yes, but I suspect it will not be strong enough to hold over the long-term.
I think I'd drill holes in the door and bolt the hook to the frame instead of trying to use adhesive or epoxy.
The best would be bolts through the entire thickness of the door with a washer and nut on the other side.  Next would be some self-tapping screws suitable for plastic (usually wood screws will do the trick here) but you'll have to be very careful to avoid stripping the plastic when tightening.