Bind the 'keydown' event to the editing box. if the user press the DOWN key, call the 'showPanel' method to display the drop-down panel.
$('#dg').datagrid({
onBeginEdit: function(index,row){
var ed = $(this).datagrid('getEditor', {index:index,field:...});
if (ed){
$(ed.target).combobox('textbox').bind('keydown', function(e){
if (e.keyCode == 40){
var cc = $(ed.target);
if (cc.combobox('panel').panel('options').closed){
cc.combobox('showPanel');
}
}
})
}
}
})