I may have found somehting :
in wp-admin\include\meta-boxes.php, search <p> markup with the class
class="category-add wp-hidden-child"
on <input type="button" add a onclick :
<input type="button" onclick="refresh_taxonomy_field_<?php echo $taxonomy; ?>();"
after this, add a JS :
<?php
echo '<script type="text/javascript">
function refresh_taxonomy_field_'.$taxonomy.'() {
var new_val = document.getElementById("new'.$taxonomy.'").value;
var acf_field = document.getElementById("acf-field-'.$taxonomy.'");
var opt = document.createElement("option");
opt.value = new_val;
opt.innerHTML = new_val;
acf_field.appendChild(opt);
}
</script>';?>
not very clean, but it works !
