Thank you for getting in touch,
There is currently no direct way to do this in Unleashed as products cannot be auto-assembled upon being receipted in through a PO. However, we would suggest setting up a warehouse for your third-party manufacturer, purchase the components in via a PO and choose that warehouse to receipt in. Your own components would also be held in this warehouse, making a warehouse transfer when needed.
Then the assembly would have as a source warehouse the third-party warehouse and destination either your selling warehouse or an intermediary warehouse as you mention that these are sub-assemblies. In the supplier costs tab of the assembly you can then add any extra charges the manufacturer might invoice you for production.
I hope that helps,
Do not hesitate to contact us at email@example.com if you have any further questions.
Your Friendly Unleashed Engagement Team