Sadly, you can't extend the the connection without replacing the entire cable which would be prohibitive. You will need to replace the antenna unit within the display lid.
We have had a lot of dialog on the pro's n con's of swapping out the older n board with Apples newer ac board (first generation). We did some tests and the amount of gain in performance was not much. Others also reported the same results.
The next generation of ac hardware is just coming out. I would wait to see what these look like. Or, look at an external unit which will give you the needed matched antennas.