Yes, you can create Email-based users within the User Management section of the platform if these users have not been synced from your Azure AD.
Be mindful that the Email-based Users domain cannot be the same as the accounts that utilise Single Sign-On, as this will conflict, and the Email-based users will not be asked for their password, therefore preventing login.
If you require further information on this, feel free to reach out to support@metacompliance.com.