spot welder attachment for arc welder?

You can plug weld with the various arc processes, that's pretty much a spot weld. If there's a big enough difference between the thickness of the two materials you wish to spot weld you can just get a quick puddle on the top piece with a short arc and it'll fuse nicely to the lower.

I remember a tool from a place I worked at, it welded studs onto your work.. the stud had some aluminum-based fluxing/shielding stuff on the end of it and it'd start out touching the work when the current flowed. Then it'd pull the stud back to establish an arc, then it'd cram the stud into the puddle it had made if you got the arc time right. Pretty futile thing, the weld was very hard and porous, didn't take a lot to knock the stud off.

John