Yes, this could be done with BPM, but the challenge is to get the email to have something that can automatically respond with buttons that actually do the approval. #AutomationStudio can make this a much more elegant solution with pushbutton approvals directly in the email. To see more, watch the video found here. This was an Insights Promo, but it still applies: Automated PO Approval (Via Email)
The same type of workflow could be created for PO Requisitions, but you could also customize the approval cycle with custom approval table rules in Automation Studio.