We sell it to the supplier on a customer order.  Set them up as a customer and deliver it on a customer order.  If you are sending it to them so they can assemble and send it back to you, then make sure to setup a pricelist that has zero markup so that you aren’t making a profit on the transfer which in turn inflates your cost when they ship it back to you. 

By definition the supplier-owned setting is used for externally-owned stock that is supplied with the intent that once received into stock, will be converted to company-owned. The supplier-owned ownership is used as a temporary ownership between the arrival and receipt into stock if the value of the Ownership Transfer Point list in the Company/Distribution/General tab is set to Receipt into Inventory.

Stock could only be supplier-owned on an arrival or quality assurance location.

If we can sell the stock through a customer order and receive back through a purchase order (using order code - external service order in the purchase order), may be we can keep them in arrival or in quality assurance with supplier ownership. Didn’t not try and verify whether it is possible.