Re-save comma separated list of values into a "multiple instances of this field" field. WPAI.

functions.php

add_action( 'pmxi_saved_post', 'post_saved', 10, 1 );
function post_saved( $id ) {
    $comma_values = get_post_meta($id, 'my_custom_field', true); 
    $explode_comma_values = explode( ',', $comma_values );
    delete_post_meta( $id, 'my_custom_field', $comma_values ); //Clear custom field first
    foreach($explode_comma_values as $v){
        add_post_meta( $id, 'my_custom_field', $v );
    }
}