Hi quietman,
Wellcome to our forum.
Yes it is possible to convert a standard welder to serve as a MIG.
Depending on the welder type and specs it can be a smaller or bigger job to achieve. One thing which affects the mod is if the welder at hand is AC or DC.
This guy Jan has built a MIG but from scratch, it might give you some ideas: http://home8.inet.tele.dk/jan_p/welder/mig1.htm