Purchasing - Item Category set used in Assignment set

  We are setting up sourcing rules for items ordered internally via i-Procurement. We want to have 2 very simple rules per organization. For non critical items source from the central warehouse and for critical items source from the local warehouse. Even for critical items we may have hundreds of SKUs.

We had hoped to do this by using the Category – Organization combination in the Sourcing Rules Assignment screen but the default category set is Purchasing. We wanted to use a new category to get the split between critical and non critical items.

My question is: Is it possible to have the Assignment Set consider another category set other than the Purchasing category set??