Server Actions can't be used to alter base fields like vehicle_type.

You would need to write a module. 

You might consider using Odoo Studio to make your own custom field though, since you are likely to be equipped to be able to do this easier than the alternative. See an older video (still applicable) at https://www.youtube.com/watch?v=o7xVFypSbLk to learn how to do this.

Developer documentation is at https://www.odoo.com/documentation/15.0/developer.html and there is a link at the bottom "Building a Module" that you should spend time reviewing.

Some review of the existing code would also be needed before using selection_add since there are places where the selection option is used for other purposes, see https://github.com/odoo/odoo/blob/15.0/addons/fleet/models/fleet_vehicle.py#L253

All this to say it is not trivial to add the additional option, so we would recommend your own custom field instead.