#1
Can you simply put a pickup cover from one humbucker onto another by just screwing it in?
#2
It doesn't screw on. And more importantly, unless you've got some spare beeswax/paraffin wax, a double boiler and a soldering iron handy, you're going to get some horrible microphonic feedback when you up the gain or volume.

Put simply, if you fit a metal cover to a pickup, you should wax-pot it first or there's a very high chance of microphonics. Google 'pickup potting' for more.