As Six Sigma practitioners would say, “Anything with a process, you can apply Six Sigma.”
If you really think about it, everything goes through a process, and so you can say that you can launch Six Sigma programs in any of your processes. However, there are cases that Six Sigma tools are not the right ones to use for a particular process. So you have to exercise better judgment. 
In any case, Lean Six Sigma, as the marriage of Lean and Six Sigma, can also be used to address issues on any process. An article on iSixSigma has a good discussion on how Lean Six Sigma solves inventory cost problem.
Manufacturers can use Lean to transform their processes from making for stock to making to order. To do this, production lead time should be less than the customer’s expectation for delivery time. After implementing Lean tools, practitioners can use Six Sigma to reduce variation in the inventory process.
