Need to create a SepaMandate for street walkers?
To generate these required Sepa mandate reference codes I thought I
  could use the API SepaMandate with the create action. This API
  requires an entity table and id, which I do not have yet. 


There's currently two API calls for creating a SEPA mandate: SepaMandate.create and SepaMandate.createfull. The first one requires indeed a contribution/recurring_contribution to exist, the second one will create both in one go. Both of them take a reference parameter with an externally generated mandate reference number. If this is not set, a new reference number will be generated. Does that answer your question?