Rainer,
Unfortunately, I am not able to provide a good answer. I attempted to locate a standard field in SAP to store tax numbers related to Canada agencies and in particular Quebec, but I found nothing conclusive. Your options are to use standard fields like tax number 1, tax number 2, etc. You can also consider adding a custom field to vendor master. SAP provides a feature called "customer own fields in vendor master" that you could adapt to serve your purpose.
John