Pretty straightforward, really.
Just get the extra length of wire that you will need. 16 gauge is more than adequate.
It's probably be easiest if you could get a piece of the same type of wire for easier color coding, but if not, then just keep track of the wire colors as you're extending them.
I think if the install was done using the CEA wiring color codes it should be:
Right + = Red
Right - = Black
Left + = White
Left - = Green
How ever you choose to extend the wires, ensure they connect to the Surround Back/B terminals of the receiver as indicated in the list above. Of course, this assumes the installer followed the CEA color code.
Then, in the receiver setup, you'll need to set the Surround Back speaker configuration to Zone 2.
Just because there is a knob doesn't mean you should turn it.