Ok, i've found out the solution, very simple: The function "ExpandRows()" will trigger a click on all (+) buttons. It's called from the img inside the HeaderTemplate on the GridView.
Thanks anyway
function ExpandRows() {
$("img[src*=plus]").trigger("click");
}
<HeaderTemplate>
<img alt="" style="cursor: pointer" src="img/expandAll.png" onclick="ExpandRows();" id="totalControl" />
</HeaderTemplate>