If you wish to use any Google Analytics functionality not already provided with our version of the GA tracking code, you'd need to stop using our GA code entirely and start using Google's code instead.
This means you would give up the ability to track cart button clicks and use GA Ecommerce Tracking, both of which depend on our version of the GA code.
When you use Google's code, you would also need to add that code to your thank-you pages, as we only insert our version of GA code when we detect that version of code has set a GA tracking cookie in the buyer's browser.