Purchase assemblies - we supply the raw materials

We sell assemblies and send the raw materials out to a contract manufacturer for assembly.  I'm working on cleaning up our inventory system in NS and realize that our current method of purchasing the assemblies doesn't decrease raw material levels.  This makes sense and I understand why it doesn't.  My question is if there is a way to purchase an assembly and somehow tell NS that we are supplying the raw materials so inventory will be adjusted.

The other approach (and possibly the correct one) would be to change from purchasing the assembly to purchasing an "Assembly Service" item.  This would require me to enter an Assembly Build transaction everytime we receive finished assemblies back.