I don't think that it's possible. I believe that it's the AT&T SIM cards that are what make the phone able to find the UMTS/HSDPA signal. A Dobson SIM will have no idea what that even is or where to find it.
Just trolling around the old forums. You can use a Dobson SIM in a 3G phone and it will work fine. It just has to be unlocked. I kept my Dobson employe plan as long as i could, and had my SIM in a bunch of unlocked phones on 3G, including iPhone 3G which worked awesome.