Actually, you just need to find two wires, cut the wire on your power adaptor, and join the two wires to the wire on your power adaptor. You can then use sticky tape to tape up the joining. A bit unsightly, but almost zero cost.
looking into past post, hope you all don't mind my penny worth opinion..alternative to using the sticky tape, it is possible to do connect the two wires safely using the 3-pin plug and socket method. Tape can come loose and may be cause some risk.
1) strip the power adapter wires and screw it into the screw's post of the electrical plastic outlet (come in two flavours i.e. single and double, use the single one).
2) strip the extended cable and put all wires into a 3 pin plug. (guess there will not an earth wire).
3) strip the other end of the extended cable to your required plug.
it will be good to give some pict so that mayb i can advise better.
disclaimer clause applies..

so do it carefully and observe a few things:
1) Prior doing, know where you electrical circuit breaker is
2) Do not do it at night. it will be troublesome to find a torch.
3) be safe and don't do short cut.
4) wear rubber slipper at home.