stick to mig

Stick welding requires a constant current power supply. MIG welding requires a constant voltage power supply. The two are electrically very different. The major electrical difference is the way the transformers are built. A constant current transformer has much more magnetic reluctance designed into it than a constant voltage transformer. It isn't practical to try to modify a constant current transformer to be a constant voltage transformer.

There are ways to make a spool gun work on a constant current power supply, but they aren't simple or cheap. Basically, a circuit is required which senses voltage, and dynamically varies wire speed to keep the voltage constant as stick out varies while moving the gun. In other words, the circuit attempts to undo what the constant current power source is trying to do.

Miller sells a separate circuit box to be used with their 30A spool gun, for about $1400, which does this. Readywelder sells a spool gun with the circuit built in for about $500. They work, but frankly you're better off buying a designed for MIG constant voltage power supply in the first place.

Gary

The device is available from both Miller and Lincoln. It is a wire feeder which can be connected to either a CC power source (stick welder) or a CV power source. Miller calls it a voltage-sensing wire feeder, models 8VS and 12VS.

If you were to make your own, it would work, just not very well as a hand-held MIG.

The practical problem is that it costs as much or more than a whole standalone MIG machine.

Practically speaking, only if your "stick welder" is really a multi-mode welding powersource. If there is a switch for CC-stick (constant current) and CV-mig (constant voltage), then you're ready to hook up a spoolgun. A normal "just a stick welder" is Constant Current only, and MIGs poorly.

A suitcase feeder from Miller or Lincoln, or a Readywelder spoolgun.