How can i put a The driver for default in all the cars without seatbelt?

You cannot do that, unless you go editing all default configuration, which is not recommended as you'll break scenarios and so on.

 

You could seriously break all scenarios, campaigns and so on by doing that. I don't recommend it at all.
I would rather suggestion simply load the vehicle you want, add the dummy part, and save it as your own configuration.
If you enable the setting to show custom configuration in the vehicle selector (in the game's options), you just click on them instead.