Budget pre-reg and additional drivers

[Cochise]

My sister is going to use the Budget pre-reg process but we want to add me as an additional driver (we may swap vehicles for a day during the holidays) but i will not be there as they arrive a week after us. Can this work with the pre-reg, we accessed it and there doesn't appear to be an option to add a driver. Has anyone managed to make this work?

Cheers