Short answer: You can’t.
Long answer: Jigoshop is designed to only allow checkout of our “product” CPT.
In reality, you should have been looking at Jigoshop’s template_functions.php file which has our add to cart functions. For security reasons, Jigoshop has checks, absolutely everywhere that the CPT is in fact Jigoshop’s own product CPT.
Source: I’m a Core Developer at Jigoshop (it doesn’t get more official than this)