I have been PMed several times lately for information on using a paintball tank to use in kegging. Yes, I have used this sucessfully for over a year.
My gallery has a photo of my setup. It has a modified fill valve for a regulator adaptor. This was not sucessful. After the second use, the fill valve gasket started leaking. This would not be a big issue for filling tanks but for us the leakage would be unacceptable. I now have an adaptor from MoreBeer and though pricey, it has no valve to leak. I will try to get a photo of the new system up later.
The best regulator adaptor for a paintball tank I have found is carried by MoreBeer. You thread out the other adaptor and screw this in with a little teflon tape. These are left hand threads and are typically very tight. Here is the link to the adaptor - MoreBeer Item: D1062
To buy a regulator, the best route is to buy single guage because you are not having to pay for the high pressure guage. It is not needed on a paintball system. I bought mine from Micromatic. Item Number 841 at $37.95 is the cheapest. I bought item 641 because I wanted the thumb screw so I wouldn't have to get a screwdriver each time I wanted to adjust the pressure. It is $43.95
MoreBeer also has regulators and in that case secondary regulator D1067A would do. You would get free shipping and the price would not be much more. I prefered Micromatic because of the thumb screw.
Again a caution about removing the tank adaptor. Mine came very tight. I very carefully used a vise and a pipe wrench to get the old adaptor out. The new adaptor is nylon and you don't have to get it too tight to seal. The factory definately overdoes tightening it on. You might call the factory and see if you can get one without that adaptor or if they will take it out for you.