Hello @StefanVollbrecht,
In short, what you want is possible 
The first step is to make sure what the technical name of the customized SSRS report is.
Then you need to register this report so that you can combine the existing SSRS design with the Docentric print destinations. You can read more details about this in our documentation:
Most importantly, you need to enable the highlighted flag Use only print destinations to Yes in order to skip the use of Docentric templates for the specific report:
If you have any further questions, please let me know.