Dear @RolandGrund,
we you can find additional information about the process here: Extensions and Integrations Guide.
I’ll quote the most interesting part:
If your working environment has limited internet access, another option for installing KNIME extensions is available. To access this additional dialog use the File → Preferences → Install/Update → Available Software Sites command. This tool has a different interface that allows update sites to be manually specified. Clicking “Add…” will allow you to either manually specify the URL of the KNIME Update Site or use the offline (.zip) version of the Update Site to customize your KNIME Analytics Platform installation. Note: It is important not to unzip the archived KNIME Update Site file prior to specifying it as an update source for KNIME.
Best,
Stefan