jQuery Grid Plugin - jqGrid

i'm kinda new with jqgrid..i would like my colmodel to be dynamic based on the data received from the xml

my colmodel looks like this.

 colModel: [{name: 'id',index: 'id',width: 120,key: true,xmlmap: "invoiceID"},
                  {name: 'value',index: 'value',width: 150,sortable: false,xmlmap: "invoiceMenu"},
                  {name: 'unit',width: 55,xmlmap: "invoiceAaa"},
                  {name:'get',label:'get', width:60, align:"center", editable:true,formatter:'checkbox',edittype:"checkbox"}

                ]

My xml looks like this

<?xml version='1.0' encoding='utf-8'?>
<invoices>
    <request>true</request>
    <currentpage>1</currentpage>
    <totalpages>1</totalpages>
    <totalrecords>6</totalrecords>
    <invoice>
        <asin>12385</asin>
        <invoiceID>1</invoiceID>
        <invoiceMenu>Tabelle</invoiceMenu>
        <invoiceAaa>1a2</invoiceAaa>
        <Get>true</Get>
        <level>0</level>
        <parent>NULL</parent>
        <isLeaf>false</isLeaf>
        <expanded>false</expanded>
    </invoice>

<invoices>

i want my GET column to be either editable or not editable based on the xml tag <Get>true</Get>

please help me