Unfortunately no, WiFi only iPads cannot be "upgraded" to use 3G. If you have a cell phone from a carrier that offers "tethering" you could use your cell phone as a WiFi device that your iPad can use for broadband access. IIRC Verizon charges an additional $20/mo. for the additional service for example.
That said many cellular carriers offer portable 3G/WiFi devices. Like a cell phone, they come with a monthly fee and/or contract. Check with the cellular providers in your area to see what they offer.
Hope that helps and let us know how it goes!
EDIT: D'oh! Gabriel beat me to it and more succinctly.