How have new users added to MC

We use a plugin (Oliver POS) for our in store POS system which allows our in store sales to connect to our websites for inventory management. These POS sales customers aren’t being added to our MailChimp database via the API, I assume because technically they aren’t going through WooCommerce Checkout which is the API trigger for online sales to be added. These new customers ARE being added as users to the website though. Is there a trigger we can set up that would have these off line sale customers added via the API?