Extenders in Case Blue-GBII modules

I know this is probably a silly question, but I cannot find how to create extenders in the Case Blue-GB II modules - either of the latest iterations. Trucks and wagons don’t have a “flip” option on the right click menu, and I can’t find extenders of either kind in the counters list.

I had the same problem.
You just have to select “Increase size” on a truck/wagon of size 5.