How about 50 F1.2 AIS? No adapter needed!
If such adapter even physycally possible it would cost $$$.
Interesting idea though. Say, you would be able to use the same screw in adapter for all 52mm filter size lenses. There are WA adapters that make your lens into a fish, sort of.
My understanding is that it's impossible to add "speed" by adding glass, you can only lose it. It's possible to put an 0.7x or similar converter on the front of the lens, but good luck getting decent image quality out of them, and you'll lose effective aperture, not gain it.
Yes; 'tis called the Nikon Lens mount. FX lenses fit DX cameras, no problems, but, depending if it is the latest thing from Nikon or not, it may not meter or focus on the DX body.
You look at the back end of the lens, and if it has like 10 contacts, it will probably be O.K., but if you have the older 5 contact lenses, there may develop a problem with focus or metering on newer bodies.
I have the D 50, and It works fine with my 20-plus year old D lenses, as well as the new ones. I hear the D3100 and D5100 are a bit picky.