How can I extend a circuit so I can plug in my under cabinet lighting?
There should be no problems with extending the circuit as you suggest. Install another box where you need it, run the cable between the boxes, and hook everything up.  I'd contact the local building department, and ask them if they would consider cable run on top of cabinets to be "subject to physical damage".  I wouldn't think so, but not all jurisdictions are the same. If they say it is, then you'll simply have to use conduit to run the wiring between the boxes.  

You'll also want to ask if they consider the cable "supported", by running it along the top of the cabinets.  They may tell you that you'll still have to secure the cable/conduit with staples/clamps.

Aside from those minor possible issues mentioned, I don't see any problems with extending the circuit.