If they are not allowing it then maybe try changing it a bit to reduce mileage. You could try PRG-FRA-HKG-ICN to knock of 1000 miles and then get a cheap return from HKG-BKK for the stopover.
Also like Jetstreamer suggested maybe start your journey in a Z2 country with a direct flight e.g MXP-BKK-ICN and get a cheap flight from PRG to MXP.
The strange thing is I imagine they would be more likely allow MXP-BKK-ICN even though it is about 800 miles further.