For ships with built-in UMMU support like the DGIV and XR series this isn't possible. It's hard-coded into the .dll. Unless you have access to the source code and compile custom modules with modified parameters you're stuck.
If you're using UMMUFA, I believe some older versions maintain a seperate config for each specific type of vessel being used, and you could maybe modify that. Otherwise, UMMUFA comes with source code, and you can modify and compile a custom version. It would be difficult, and may not work with every ship.
Are you having a problem with MMU's appearing quite a ways away from a ship?