The AFCAD file format doesn't allow for more than 4 characters in the atc_parking_code, the only possible way is to create a fictional 4-letters ICAO for Menzies and then changing all the sim.cfg of the GSX vehicles.
But I don't recommend this solution, both because sim.cfg files might be overwritten by GSX updates, but also because we'll shorty add a feature to allow to freely select the ground operator, from a menu containing all possible choices, sorted by score.