I think the employer has a fair amount of leeway in what they offer, so they’re going to be the ultimate authority on what the options are as far as Roth vs. traditional contributions and rollovers.
I do believe that you have no obligation to roll an old work retirement plan into a new job, though. I think the usual play is to roll the old Roth account into a regular Roth IRA (like through Vanguard) that you can control.