I'm planning to use a long range USB RFID reader (12V) like this with Pi. As Pi USB can supply only 5V, how can I connect it to Pi USB, if it is not external powered..??
A switching step-up converter could be used to provide a higher voltage (12V) from a given lower voltage (5V). Be sure that the 5V power supply of the RPi can provide high enough a current though to drive that additional load.
ElectricalEngineering.SE might however be better suited to help on that matter and recommend a circuitry for that need. If you just want to buy some ready-to-use gagdet look out for "DC DC Step Up Converter".