Thank you for posting. I'm not sure if I'm picturing your desired result correctly, but maybe I can give you a few tips to point you in the right direction.
First, PB does have many importing and exporting capabilities. You mention that you have an existing Excel file. We have a video on importing from Excel at http://www.thebrain.com/support/tutorials/import-and-export/word-excel/
that you may find helpful. This will simply copy the existing hierarchy into PB, from here, you can manually add any desired cross references and additional attributes (Types and Tags) that a standard Excel file is not capable of displaying.
If you're looking for something much more technical, we do have the ability to import/export properly formatted XML files. I have attached a document that you may find useful.