You can add extra icons with multiple class name. The code below shows how to add an icon with 'navi' class, which is hidden and only show when hovering on it.
<style type="text/css">
.navi{
opacity: 0.0;
}
.navi:hover{
opacity: 0.6;
}
</style>
<script>
$(function(){
$('#cc').combobox({
icons:[{
iconCls:'icon-search navi'
}]
})
})
</script>