Hi there!
Thank you for posting in the forum.
When you install the our WooCommerce MailerLite plugin, your existing subscribers in MailerLite will have their purchase data synced.
Additionally, all new subscribers that agree to join your mailing list will automatically be added to MailerLite.
But unfortunately, customers that already existed in your WooCommerce list but did not already exist in your MailerLite list will not be synced, along with new subscribers that sign up without accepting marketing emails.
If you want to export subscribers manually from WooCommerce to MailerLite you can use third partyplugins like this: https://www.webtoffee.com/export-customer-list-woocommerce-shop/ or check with Woocommerce a way to export existing customers.
But this can be done only for people who provided you with consent that they are happy receiving emails from you.
Let us know if you would have any questions.