According to the Salesforce forum, you cannot use SOQL to query custom reports.
The easiest way to do it is via API. You would need to extract the connection information using the Salesforce Authentication extractor. With this information, you can then make authenticated API call for salesforce within KNIME.
You might find useful this link with Salesforce documentation of endpoints related to reports. There you can see which endpoints you can access and what kind of information they output.
Let me know if this helps.