WP All Import / Update stock quantity from multiple XML files

I managed to get it working, here is answer:
– when importing update new stock to custom field: _custom_stock_placeholder
– update only this field: _custom_stock_placeholder
– add this code to functions.php

add_action('pmxi_saved_post', 'post_saved', 10, 1);

function post_saved($id) {

$original_stock = get_post_meta($id, '_stock', true); 
$new_stock = get_post_meta($id, '_custom_stock_placeholder', true);

$combined_stock= $original_stock + $new_stock;

update_post_meta($id, '_stock', $combined_stock);

}