Es gibt in der Tat eine Möglichkeit eine Betankung in den Flugplan einzubauen (STAYn + STAYINFOn). Jedoch ist es für Luftbetankungen vorgesehen, siehe unten.

STAYn + STAYINFOn/
Operations in a STAY area are expressed using free-form text (n = number).
Use of the STAY indicator in IFR FPLs sent to the IFPS.
The STAY indicator may be used to indicate the time in an area (defined as the
‘STAY area’) and associated with certain special en-route activities such as
training flights, air-to-air refuelling, aerial photography or calibration flight.
Instructions for using the STAY indicator