You would need a 500K pot. Adding a 250K fixed resistor to a 250K pot, either in series or parallel, will not give you the control behavior you're looking for. But before you order one, I'd install the 250K you already have coming, and see how it sounds to you. If the humbucker doesn't sound bright enough with the 250K, you can always change it to a 500K.

I recently converted a Squier Classic Vibe Strat to an HSS configuration, adding a DiMarzio Air Norton at the bridge. To make sure that the new humbucker is in phase with the single coils and remains hum-canceling when the coil-split bridge & middle pickup combo is selected, you'll need to know its magnetic and electrical polarity, and wire it in accordingly. I posted about that upgrade HERE and HERE. I can provide additional details, if needed.