Thanks Marius, that lifted some of the fog already!
Would there happen to be some example or production code that uses the API that I can look at? Because now I'm having some problems figuring out the parameters and particularly their allowed values. E.g. sites_web_domain_add() gives me an "hd_quota_error_empty" error when hd_quota is definitely present but '0' in the SOAP request.
Other than that, the converter is coming along nicely. Still very unpolished but doing its job. Gonna test it on some real installation and then release it in the coming weeks.