Assign a work item template entry to an iteration?

I am customizing a process, and we have a number of "standard" activities that must be done at different particular iterations of a project. I can create the plan items for these activities by going into the Process Configuration -> Project Area Initialization, and creating the appropriate work item template entries. What I would like to do is to have these assigned to the specific iterations that they belong with. When I create these work items, they all come up with the Planned For field set to unassigned. Is there any way that I can specify default field values in the XML in the Process Configuration Source?