Hello,
Unfortunately, you can’t add login function on Registration page via admin panel. You can try to implement it only by additional customization in case you have appropriate skills.
If you want to leave one page with both registration and sign in form – you can do this:
– Go to WooCommerce > Settings > Accounts > Enable registration on the “My Account” page (and registration form will be displayed on Sign in page);
– Go to Pages > delete Registration page (and Register link will disappear from header).