Assigning an additional user role with Woocommerce purchase

Woocommerce by default assigns a role of ‘Customer’ to everyone who purchases a product from my website.

I have many user accounts who have not purchased any items via Woocommerce (they have been set up manually) as such they have a different user role.

Is it possible so that when an existing user goes through the Woocommerce checkout they are assigned the ‘Customer’ role in ADDITION to their existing role?

Right now the customer role is only being assigned to new users who go through the checkout and not existing users.

Thank you!