jQuery Grid Plugin - jqGrid

How can I post some other parametres, when I initialize the treegrid this way

var reviewGrid = jQuery("#treegrid").jqGrid({
        url: 'server.php?q=tree',
        treedatatype: "xml",
        mtype: "POST",

        ...

this parametres are not in the treegrid table, but for taking data the are necessary, adn they must be sented with the parametres.

I think, Module name: grid.postext.js can help me, but where and when I must use

jQuery("#grid_id").appendPostData( newdata)

Please, code, if you can.

Best regards,Satellite