Thank you for your quesiton. What do you mean by the track extension? Are you saying you want a 18ft long double door hardware? If so, we can create a one-time listing for you to make the order. Please be more specified, thanks!

Yes, per the copy: " All tracks now have additional holes at the end of both sides for expanding the track size. For tracks 10' and above, connecting tracks are now simple and quick, using a junction plate at the ends of the tracks to connect."

So I would need the 6' Flat Track and 2 junction plates to make it 18' total...

You can go to the following listing to purchase the 16' hardware. We adjusted the price already. We will make sure to send you 18' instead. Thanks!
[https://www.houzz.com/products/sliding-barn-door-hardware-for-double-door-straight-design-16-prvw-vr~63982150?lid=48296977[(https://www.houzz.com/products/sliding-barn-door-hardware-for-double-door-straight-design-16-prvw-vr~63982150?lid=48296977)

Fantastic. I just placed the order. Thank you!

Received. We will send you 18' double door hardware.