You've got an interesting challenge here - in that you are seeking to stay OUT of the SCM management system (which is why we would want an NSN in place).

Your solution is going to reside in creating a local National Item Identification Number (NIIN) that will have the requisite FSCM four-digit number at the start to align your components with the proper category of material.  The next two positions will be an "LL" to indicate it is a locally generated number.  You are then free to apply the final seven digits in whatever fashion you arrange with the contractor.

You can set up your own WAWF system to recognize these items by their Local NIIN - and pay for delivery on them - using this coding system.  You will NOT be able to access DLA services for procurement and sustainment of future spares with the Local NIIN.