To create a datagrid with only add ability, no update or delete another record is created, use enable_edit and pass "C" for Create in the 2nd parameter.

$dg->enable_edit("FORM", "C");

Then add the following to the bottom of your page before </body> tag. Here the table name used is "orders". Replace it with your own.

<script>
jQuery(document).ready(function($){
var grid = $('#orders')[0];
$(grid).jqGrid('setGridParam',{ondblClickRow: function(){ }});
});
</script>