If you have a product that is assembled on your behalf by a sub-contractor and you supply the parts to them, this can be modelled by a method consisting of a single sub-con step with all the parts specified in that step, e.g.:

The product must be tagged as “Can be manufactured”, it's still a manufactured part it's just that it's being done on your behalf by someone else. Invoking this method will cause the planning system to co-ordinate the acquisition of all the parts (be they bought or made) and then it’ll raise a purchase order to the contractor describing the work to be done and listing all the parts you are free-issuing to them.