Tech Portal: How to issue not known serial numbers (sub-assemblies of the parent item)

Gurus,

Any help is appreciated:

An assembly comes back for repair;  DR will create the new serial number and instance in IB  if install-base tracked. So far so good!
The assembly contains a serial subassembly which can be put into stores using a WIP Negative Issue on the non std job for the assembly.

Question is, how does the serial number for the subassembly get created (if it is pre-defined control)?  Manually, or is there any similar functionality for DR to create it?

At the moment in Tech Portal you cannot just type in the "new" serial number like we do in Repair Orders - you need to pick up from an existing list of values. Is there a way to make Tech Portal work like the Repair Order Details?