Stoner 63 in SOG:PF is in rifle configuration that's bottom fed from 30 round magazines and fires from closed bolt (XM22/XM22E1). The 150rd "drum" used in LMG/Commando Stoner 63 (XM207 and Mk 23 Mod 0) was actually a container for ammo belt. Since rifle configuration can't feed from the belt, the whole gun has to be converted from one configuration to another - LMG feeds from side, and fires from open bolt. Convertion can be done in field, but requires a whole lot of additional parts - that's why US military (including USN SEAL teams) issued the guns in specific configurations (XM22 rifle, XM23 carbine, XM207 LMG and Mk 23 Mod 0 Commando), and not as a "DIY kit".

What you're asking for is not the ability of "loading 150rd mags into M63" (since there are no "150rd mags" for that gun), but inclusion of additional Stoner 63 family variants, most notably the belt-fed LMG/Commando versions - and that's indeed a good request.

Some "introductory course" to Stoner 63 family of weapons: Modern Firearms 1, Modern Firearms 2, Forgotten Weapons 1, Forgotten Weapons 2.