I do not think it is directly possible to have item category wise levels in the position hierarchy. The position hierarchy can be created for different account and amount levels. I think you can achieve this by creating two separate position hierarchies and the buyer can select the appropriate hierarchy when submitting the PO for approval.
Indirectly, I think you can follow below customizations to achieve this:
1. Apply item categories to the items.
2. Modify the PO Account generator workflow to generate separate charge accounts by looking at the category assigned to PO line item.
3. Create one hierarchy based on these different charge accounts to be routed to different top positions.
As the previous responder mentioned -- its not possible to for the system to look at which Item Category is on the PO and then choose which Position in the Position Hierarchy to direct it to. You could create two Position Hierarchies and then have the user choose while approving which Approval Path (Position Hierarchy) to route the document through.