i don't know of any extension cable for the plug. you could use a standard AC extension cable at the inverter, or cut the wire and add 6 feet.
if you cut it,
unplug it before you start
just make sure you use a splice wire equal to or larger in diameter than the original wire
make sure you connect the correct wires, not pos to neg
make sure you solder/connect the wires well
wrap up the exposed wire with electrical tape