I am setting up a facility to remotely do some manufacturing and assembly of a product.  I will have a warehouse there with the general purchased items inventory.  There are sub assembly weldments that I set up as a purchased item since I am purchasing them finished from this sub contractor.  I am also hiring this sub contractor to assemble other purchased items with the weldments into a sub assembly.  

What's the best way to accommodate this in Odoo as far as BOM and manufacturing?  I set up a BOM that has the weldments in there as purchased items along with all of the other general purchased items.  Then I added an operation called "sub contract assembly" which has a cost associated with it in the bom routing.  However, when I add an operation I cannot classify the BOM as a sub contract.  I'll have to issue this vendor a PO for the weldments and assembly time, they will be using the purchased items I keep in inventory in my "remote warehouse" within their facility. 

Should I be handling this differently?  Thanks.