Was doing some more checking. Here is a link to Miller's line of multiprocess machines (Stick, MIG, TIG):
http://millerwelds.com/products/multiprocess/productguide.html

And here is an example of an add-on wire feeder:
http://millerwelds.com/products/wire_feeders/s_d_70_series_feeders/

Only problem is that the multi-process are about $3K and up while the wire feeders start about $600~$800 and go up plus you have to buy a suitable power unit (Stick or TIG) to add it onto...

My personal arsenal consists of a Lincoln AC-225C stick welder (bought used for $150 - about half price), a Millermatic 175 MIG (again, used, $500 - about $200 off), and I am looking to add a Miller Synchrowave 250 TIG. The Synchrowave will also do stick, so I'll probably sell the Lincoln at that point.