no option available to call lookup while defining approval rules for purchasing document

Summary:

While defining approval rules for BPA (task: Manage Purchasing Document Approvals), no option available to call lookups. 

We want to initiate approval for Blanket Purchase Agreement based on specific set of categories. 

But we do not want to hardcode the categories names by using 'Document Line' level attribute 'Category Name' using 'In' clause. Because this categories list will not be constant and keeps increasing in future and we do not want to visit the approval rules every time the category name gets added.

So, we want to maintain a lookup for this purpose and then call this lookup while defining approval rules.