Perhaps this is simple as making the dice roller fields optional. (though, if you set one, then you need to set all of them as required, or leave them all blank to not use a dice roller). To make the UX better for this we could default a checkbox "Use MARTI dice roller" to checked, and if you uncheck it then it hides the MARTI dice roller fields. A Help button would be pretty key here to explain that MARTI would do external dice-rolls and internal would simply just not use that.
I think there are probably some implications for the game engine itself as the way PBF is done is kinda hacky, the flag indicating PBF is not direct and probably would trigger a request to Marti. We'd need to make the 'using-marti' be an (internal) game option and to check only that for whether to use marti or not.