Bombardier was relatively better at allowing off-market customization work on the Global Express series. In fact, I think that was one of the options presented to the PN for the LRMPA, but it lacked the payload capacity. That said, in 2023, a Canadian firm by the name of PAL Aerospace proposed a major modification (very similar to what the PN intends for the Sea Sultan) to increase payload (via under-fuselage bay).
It's unfortunate that the PAF, PN, and PA didn't standardize on one platform from the onset and pool a bigger order together. I'm sure that between them and their current and future special mission aircraft needs, they'd need around 20 aircraft in total. And besides working directly with the OEM to purchase and modify the aircraft, there was also the avenue of setting up an official, licensed MRO line that would service them and, eventually, carry out modification work domestically.