Hi Daryl,
It is technically feasible to accomplish data transfer between Ariba and SAP BW using middleware such as SAP PI or Boomi.
At high level following steps needed:
1. Leverage mail adapter within SAP PI.
2. Configure mail sender to access email messages with XLS report attachments.
3. Configure mail receiver to process the file and place into target directory for additional processing.
Thanks
Sanjay