I ordered the Chrono Felix (typically in a 38.5mm case size) in a 41mm case and Richard added the GMT function to the watch. 

I don't have a WorldTimer from Maria and Richard, but you can ask them directly. It should still be possible but the wait times are now longer than usual.

Love this watch and the brand. Maria and Richard Habring are doing great stuff in Austria. Proud owner of many Habring2 pieces.