PO Item Category

Is it possible to create a new structure for PO Item Category and link it with the Requisition and PO forms so when user is creating a requisition or a PO they can select a value in 'category' field from the new value set.

We need this in a bid to clean our categories as it is too big. I cannot just go and disable the existing values as if it is used in a BPA then releases will error out and I cannot change the category in a BPA. Any help or suggestions would be appreciated in how can I clean up the category list.