This solution combines the use of two disparate features.  The first feature is intercompany shipment/purchase and the second feature is back to back orders on SO.   The subcontract site has to be set up as a site and set up as an intercompany BP to allow shipments and purchases between them..  

1.  Set the subcontractor up as an inventory site.  Set them up as an intercompany BP, supplier at least. 

2. Move your materials to the subcontractor site.

3. Execute a work order to build the items to the subcontractor.   Assume you have manufacturing.   If not, further discussion about how to do that. 

4. On the finished good set up the subcontractor BP as the default supplier.  I am going to ignore intercompany price/markup/ assembly cost in this discussion.

5. Enter a customer SO at your site. 

6. From the SO initiate a back to back PO for a direct ship to the customer and send it to the subcontractor site (send is not needed it will just appear since it's intercompany and they are the default but it's of course in X3 where they probably don't have access). 

7. This will initiate and create the SO to ship from the intercompany subcontractor site to the customer site.  

8.  When the shipment or receipt is completed it should close the loop on the intercompany transaction.   This step is fuzzy right now.  I can't remember if it closes it when the subcontractor ships the SO or you have to receive the PO back to cover the SO.   Have to walk through this again.   I think the site shipping - subcontractor - close there SO which closes the PO which fulfills the original customer SO.