Linking Form And Subform

Firstly, apologies if any of the following seems to denote bad database design or a lack of forward planning. It does, but I've inherited it, so I'm stuck with it (for the time being).

I have a Subform (Training_Records_Sub) linked to a Form (Employee_Training). Both forms get their data from tables of the same name.
Up to now, these have been linked on the field Employee_ID. This has been working fine, as all information stored related to permanent employees, so everyone had an Employee_ID value.

Now, however, I've been informed that the system also needs to store information on temporary and contract staff, neither of whom have an Employee_ID number. In fact - for reasons I fail to understand but which I'm sure make sense to management - there is no unique indentifier stored for any of these people.

The best I can come up with is as follows:

Permanent Personnel - Can be uniquely identifed by Employee_ID and Cost_Center.

Temporary Personnel - Can be uniquely dentified by Last_Name, First_Name and Requisition_Number (this last number can apply to many people). I realise that two people of the same name COULD be hired under the same requisition number, but I've been assured that this won't be allowed.

My problem is in working out how to now link the Form and Subform.

Is there some way to use an 'If...Then..Else' statement? Even in VBA? I've sort of got an idea of what I need, but a day of playing with the code hasn't worked, so I thought I'd try posting.

Any ideas greatly appreciated.