If you select the Default Credential Provider Chain local ENV variables or the credentials file should work. Looking into the aws-sdk-java, session tokens should be supported too, but i have not tested it so far. Do you use IAM roles?
This is only the representation inside of KNIME. What Node do you use access S3 after the S3 Connection node?
This should work with the Default Credentials Provider Chain. Can you test this with ENV variables (AWS_ACCESS_KEY_ID, AWS_SECRET_KEY and AWS_SESSION_TOKEN)?