I am not one to say " you can't do that....or it won't work" because I hate it when people say that to me, but...........
It wont work.
As bill says in his post, your machine is a constant voltage machine. I have "cheated" with making a "tig" weld with my mig welder before. a real cheap and easy way to do this, is to take a contact tip, slice it down the middle and bore out the hole to accept a small tungsten, so that when you tighten up the tip, it grabs the tungsten kind of like a collet does in a tig torch. Use pure argon for a shielding gas, (obviously release the drive rolls or remove the wire altogether), use DC electrode negative, and scratch start the tungsten when you pull the trigger on the mig gun.
Having said this, you will get a tough to control welding arc with not too great characteristics.....
The other problem with your machine is the fact that it is DC only. I am sure that you could rig up some sort of high frequency device if you try hard enough and have the right parts, but being a constant voltage machine, I believe this is a waste of time.
Like I said earlier, I hate going to a store and having the guy behind the counter tell me that what I am trying to do won't work..... I usually tell them to just sell me the parts that I want, and I will figure it out for myself......but this is a case where if you really want to try it, good luck.
Sparx