Assign Employee ISSUE through Maintenance User Workbench

Dear Experts

I am using Maintenance User Workbench responsibility with EAM:Supervisor profile option on.Thus it allows me to use Assign Employee button in User Workbench itself.

When I click Assign Employee button I get a page where I can change totally department and/or resource and/or employee.

Requirement was to allow to change the resource and assign relevant employee to Work Operation.I though Assign Employee Page as described above will solve my requirement.

To my surprise when I click Assign Employee button,the result page allows me to Search Different Department and/or resources and/or employees but I cant assign work to employee other then the resource which is already attached to Work Order.