Changing "old" Gateway Service in SEGW?

Hi,

I am in the situation that I have to change an existing Gateway Service that initially was not created using SEGW.

Ideally I would like to change this service using the transaction SEGW. But how would I "import" an old service into there if not project was created for it during creation?

In case such an import is not possible:

I also can't find the option GW Data Model in SE80 where I would change the service "the old way". Any idea what the reason might be for that?

Thanks and best regards,

Daniel