function add_custom_meta_field( $meta_fields ) {

    $membership_levels = get_posts(
        array(
            'post_type'  => 'llms_membership',
            'nopaging'   => true,
            'fields'     => 'ids'
        )
    );

    foreach ( $membership_levels as $id ) {

        $meta_fields[ 'llms_enrolled_' . $id ] = array( 'label' => 'Enrolled in ' . get_the_title( $id ), 'type' => 'date', 'group' => 'lifterlms' );

    }

    return $meta_fields;

}

add_filter( 'wpf_meta_fields', 'add_custom_meta_field', 20 );

function added_to_membership( $user_id, $membership_id ) {

    if ( function_exists( 'wp_fusion' ) ) {

        wp_fusion()->user->push_user_meta( $user_id, array( 'llms_enrolled_' . $membership_id => time() ) );

    }

}

add_action( 'llms_user_added_to_membership_level', 'added_to_membership', 10, 2 );