I am glad to hear that you like the node.
Currently, it is not possible to define the relationships in the node. We are now discussing this feature and I will keep you up to date (maybe we will manage to get it implemented for the upcoming release).
There is a kind of cumbersome workaround to allow you to create datasets with relations right now. You need to create the dataset with all tables, columns and relationships in advance with the API (Note the “Try It” button on the REST API reference page). Then, in KNIME, you can configure the node to append to the existing dataset.