Registration of the truck in a separate slot [Question] (Euro Truck Simulator 2)

Greetings to all modders.
That is:
Let's say I downloaded some truck, I have not password-protected file *.scs
What you need:
I want the truck replaced some default model, and to give him a separate slot.

How can I do this manipulation? If you can write step by step. Well, or throw off the link to such a guide. Thank you.