Hi Marcel,
Ariba does provide ABAP Transports (which can be found on connect site) to integrate with SAP using either Webservices or ITK. You can modify these transports as per your requirements to extract the master data from SAP.
When integrating Ariba with SAP using ITK, to extract these master data from SAP, you will have to schedule jobs in SAP, which in turn will place all extracted data (csv files) into the specified system folder in your server (where ITK has been installed, generally ITK will be installed on the same server where SAP resides) to be consumed by ITK's Data Transfer Tool to load the same into Ariba.
Hope that works.
Thanks,
Sandeep Jain