How to Configure a Maintenance work order approver by using a descriptive flex field value

Summary:

Client has a requirement to send maintenance work order for approval. We were able to do that through configuring rule in BPM. E-signature used for this and however client wants to select approver at the point of work order creation and not with a pre defined or fixed approver. For this we enabled a Work Order DFF field with User value set and our intention was to map that here. However we cannot find dff objects in condition list. We need to know whether this DFF can me mapped to condition in BPM rule or is there a