Rema - I've done this before via a requisition import. As approvals are already done, just have the req submit in Buyer with no approvals.
Ariba has constructs in place to allow Req Import.
Are you using Ariba in the cloud or are do you have it installed on-premise? On-premise, look for the CSV files RequisitionHeader.csv and RequisitionDetail.csv and run the RequisitionImport data pull.