This can be done by a GUI extension. Simply download the attached zip folder and unzip it. Place the included .CarMaker.tcl file in your project directory on top level. As soon as you restart CarMaker or choose this project, there will be a new Add-ons button, which gives you the possibility to start your executable by clicking on ExternalGUI. In this simple example, the Windows Editor (notepad.exe) is only used to show the functionality.

To change this default, simply edit the path to the executable:
open |C:/WINDOWS/system32/notepad.exe

Download: GUI_Extension_StartProgram_withButton.zip