Thanks for reaching out. While adding items to cart is not an automatically tracked event within Site Kit there is an open GitHub issue in relation to more enhanced eCommerce features. Feel free to subscribe to or comment on this issue, which you’ll find below:
https://github.com/google/site-kit-wp/issues/459
While we don’t have any timeframe on reviewing or implementing this issue at present you can setup event tracking using Google Tag Manager or by following the guides from the below Google Analytics help center article:
https://support.google.com/analytics/answer/1009612
If using Google Tag Manager to insert your own Analytics snippet Site Kit can recognize this and use this Google Tag Manager placed Analytics snippet which include event tracking tags, as opposed to Site Kit placing a basic Google Analytics snippet.
Let us know if you have any further questions on this.
