MVC Ajax合并表单数据

I'm trying to append data to a form collection via ajax, My code is as follows:

var AjaxSendData = $('form').serialize();

$.ajax({
    data: {
        'searchtext': searchtext,
        'numofres': numofres,
        'orderbydesc': orderbydesc,
        'columnnum': columnnum,
        'pagenum': pageNum,
        'contenttype': contenttype,
        'MediaFilterType': MediaTypeID,
        'status': status,
        'ResultsFilterID': ResultsFilterID,
        'FilterIdType': FilterIdType
    },
    type: 'POST',
    url: $('#hiddenajaxurl').val() + '?' + AjaxSendData,
    cache: false,
    dataType: 'json',
    success: function (result) {
        //...
    }
});

My Issue is that on the C# side it only sees the things in ajax data: as a form collection the rest of the request is not there. I tried sending AjaxSendData as the ajax data and append to it using an Associative array ie:

//AjaxSendData['searchtext'] = searchtext;

However this cause a 500 error. Does anybody know how I can combine these so I can get the data back out of the FormCollection?