To add ACF fields to your custom registration form you would have to use acf_form() with the “form” argument set to false and you would call acf_form() between the <form></form> tags of your other form. Of course this means that you are able to call a PHP function in the correct location.
If you cannot do the above then you would need to create your own input field and then you would need to use PHP to update the fields yourself when it is submitted using update_field().
