Found out on my own after all. Hope this answer can help also other people.
<?php
$this->widget('bootstrap.widgets.TbHighCharts', array(
'chart' => array(
'borderColor'=>'#e5e5e5',
'type' => 'pie',
),
// .. 'options', 'legend', ..
'series' => array(
// --------- inner layer of the pie
array(
'size' => '40%',
'data' => array(
array(
'name' => //name
'y' => //value
),
array(
'name' => //name
'y' => //value
),
),
),
// -------- second layer from the inside
array(
'size' => '60%',
'innerSize' => '40%',
'data' => array(
array(
'name' => //name
'y' => //value
),
array(
'name' => //name
'y' => //value
),
),
),
// ----- add as manny layers as you need to
),
// ..
);
?>
Pay attention because layers are not directly linked one to the other. This means that you have to wisely set the values 'y's in order to make so that the bounds matches between the layers.