add sort types to Iteration plan

Hi,

I have created a project with Scrum template
I wanted to use the MoSCoW rule as a priority method so I've add a new work item (for Story) attribute for priority named MoSCoW_pri and also added to the layout..So it works..I didn't use the default priority attribute (1,2,3,4...leveled) so all WIs stay as unassigned..

But the problem starts with iteration plan. In iteration plan for sorting the Work Items, you can just see the default "priority" atribute of the story but I'm not using this priority attibute I'm using MoSCoW_pri so sorting does not work for me according to the MoSCoW_pri attribute..

Is there any way to use this newly added attibute in iteration plan WI sorting..

thanks..
Halil.