If you are wanting to add another category level for when you're creating products on Procure Wizard, this is done by the following:
On the left-hand side of Procure Wizard, click Product management.
Click into Manage Categories.
Select the 1st level category you are looking at adding another category to.
Select the 2nd level category this is going to sit within.
Click add category.
Give the category a name and click create.
Once the 3rd level has been created, this will show up when creating a product.