If the reason is that the car rental office does not have the right size to fit your rental car, ask if they have a larger vehicle which does have the appropriately sized accessories available (or will fit your luggage) inside.
If not, ask if another local office can deliver the necessary accessories to you.
If the accessories are not necessary immediately (say you need snow chains when you get to the mountains) suggest you buy some on the way (almost all motorway service stations carry stock), and that the hire car firm reimburses you when you return to drop off the car.
Do not fit roof racks or other accessories to the car without permission from the hire car company as you may be liable for any damage caused by their use.
Find comprehensive car hire excess insurance on the MoneyMaxim car hire insurance comparison service.
If you have any queries please do not hesitate to contact the MoneyMaxim team.