Thanks for your help, Tim! I iwill try out this method.

I have one thing in my mind how can this problem can be solved more elegantly but there are some limitations by jqGrid that does not allow to do it. There is BeforeAjaxRequest (fix me) method that is called before jqGrid sends request out to server. It is perfect place to set some more variables for request and this is ideal place where initial filtering values can be set. Problem is – it is not possible to set variable starting with underscore (_) because jqGrid ignores them.