You could use the HF box on the AC welder and turn the argon on with a valve.
DO NOT TRY TO RUN THE OUTPUT THROUGH A FOOT PEDAL. The foot pedal controlls a small ammount of power that regulates the output of Your DC machine.
If the HF unit does not have a contactor You could add one, cheapest way would be to get one from a scrap machine. You could add a gas selonoid valve too.Most likely You will not be able to have remote amperage controll, that depends on how Your AC welder controlls amperage.
If You tell Us what equipment You are using You might get more specific information.
Hello seagull369, all of the folks have given you some great answers and precautions. I believe the hi-frequency unit that you have isn't designed for AC anyway, it is likely a start-only, hi-frequency type delivery system. AC welding uses hi-frequency as the carrier to keep the arc going through the 1/2 cycles of the two different DC sides/reverse and straight polarity. The reason for the hi-frequency that is delivered by your snap-start system is only to initiate the arc without touching the tungsten to the work and then it shuts off after a few seconds. If you opt to try to use your AC 225 to tig weld with you will want to find another hi-frequency module that is capable of continuous hi-frequency, likely it will also have the start-only capability as well, there are quite a few out there, I believe Miller, Lincoln, Airco, Midstates, and a few others manufacture them. As Dave mentioned you could use one of these units to provide the hi-frequency required and use an air-cooled torch with a manual gas valve. As far as amperage control goes, you wouldn't be able to have that option with your machine as it isn't a simple matter to make it capable of a control such as that. Instead you would need to set the amperage and try it and adjust it accordingly to make the particular weld you are trying to do, in a sense the same way that scratch start welding is done with DC straight polarity for welding a lot of pipe(only minus the hi-frequency).
If you decide to try using the DC straight polarity method, you will indeed need to use helium and you will also have to clean your parts "really" well. Even then, you won't end up with the type of aesthetics that can be done using the AC current. You will be able to make a sound weld, however. Just a few more $.02's to consider. Best regards, Allan