[chris vickery]

When I used to work for our plant we did a lot of stud welding.
Basically, the stud welder was a hand held trigger device with 3 protruding prongs
Which was the ground once placed firmly on the plate. In the center there were different changeable bushings which held both a weld stud and a flux ring depending on thread size.
When the trigger was pulled, current would flow through the stud to the steel plate for a predetermined time while the flux ring would shield and clean the area similar to how flux works on an ordinary welding rod.
As far as I know, the machines we had were made solely for this purpose. Perhaps some company makes an adapter to use an ordinary arc machine?
When I am on a field job, sometimes we need to replace studs we simply use a machined up stud which has been chamfered and arc weld in place. The only issue with this is the fact that we are talking large diameter of 1/2" plus.
I think I would opt for Robin's suggestion of tig welding smaller studs in place, at least this way you could at least obtain the correct filler rod to ensure a good clean weld. Alternatively, I suppose a couple quick hot tacks with a might setup and the correct wire and gas would also work.