To the best of my knowledge, you just add the template file in the template folder in the Creo Common sub-folder.
...\Creo 2.0\Common Files\M030\templates\
This is the default folder where Creo looks for these files. But you can change this folder with config.pro which is highly recommended so you don't loose your customization when you update the S/W. See "start_model_dir" in config.pro
Once you move the new template files into the appropriate folder, you need to reboot Creo to see them. I just moved a part called my_start_part_mm.prt (remove the numeric extension) to the templates folder and after rebooting Creo, it showed up when I started a new part.