Send the custom field "last_name" to Kit.

my_pmpro_kit_add_last_name_to_subscribe_fields.php

<?php
function my_pmpro_kit_add_last_name_to_subscribe_fields( $subscribe_fields, $user_email ) {
    // Get the user by email
    $user = get_user_by( 'email', $user_email );

    // If the user is found, add the last name to the fields
    if ( $user ) {
        $last_name = $user->last_name;
        if ( $last_name ) {
            $subscribe_fields['last_name'] = $last_name;
        }
    }

    return $subscribe_fields;
}
add_filter( 'pmpro_convertkit_subscribe_fields', 'my_pmpro_kit_add_last_name_to_subscribe_fields', 10, 2 );