Create a new class for these cars, can be anything as long as unique ("a24q", "myclass", etc., will never been seen in game), add this to the class line of each car you want to use. In the series gdb file where you want these cars, create a new class, for example a24q=These Cars. Now when you select that series in the game there will be a "These Cars" menu item with all the cars with a24q in their class line listed.
If you want this to be a separate series create a new gdb file for it, name it SIM_These Cars, the first line in the file, "series=These Cars" (this will appear in the series menu), and only one class "a24q=These Cars".