Can I replace an outlet that uses knob and tube wiring with a grounded outlet?
If you need a receptacle that can accept a grounded plug but don't actually have a need for grounding, you have two options: (1) a GFCI-type receptacle marked with the words “No equipment ground,” and (2) a three-prong outlet protected by an upstream GFCI and marked with the words “GFCI protected” and “No equipment ground”. See section 210-7(d)(3) of the National Electrical Code.

If you actually need grounding, I'm afraid there are no shortcuts: you will have to rewire.  A trick to avoid opening the walls is to run the new wire in the (unfinished) basement or attic, then drill up from the basement or down from the attic into the wall where you want the receptacle.  Try to be a little more careful than the previous owner or electrician who rewired our house: we have a couple of holes in the floor because someone missed the wall!  If you rewire, you may have to obey the code's requirements for number and placement of receptacles, not sure.  In our house, the old two-prong outlets in the baseboards have been disconnected and left in place.