Hello,
Would be interesting to know more about scenario.
Why do think that you need these items as "non-stock"?
You mentioned that 3rd party produces your products.
What happens next? Who owns produced products? Are they still stored on manufacturer site? How is product shipped to customers?
My first guess would be to have a separate location for 3rd party. You create purchase order for normal items and choose that separate location.
Once product is produced you can post receipt. Now you have overview of produced products and can ship them from that location.
Thanks