Not sure if such a feature is actually desired, because a segmentation does not really belongs to the DICOM-series (and at least for the MITK segmentations is no DICOM series) , so disallowing to add additional data to a existing DICOM-series might have been done on purpose.
Besides this, you can always add resources to the patient's Resources folder or add more than one if needed.

I understand your point and have also stated this, when we were talking about this task.
The idea is to have the corresponding data (e.g. segmentation) at hand when looking at the DICOM scans. A patient's session can contain several DICOM-series so putting the additional corresponding data in a new folder (resources) outside of the scan folder would require the resource data to follow a certain naming scheme or the resource folder to have a certain subfolder structure.

Ok, I see...
you could try something like

canHaveResourceFolder |=  dynamic_cast<ctkXnatScan*>(xnatObject) != NULL;

in the QmitkXnatTreeBrowserView.cpp file around line 1120. Not sure however, if CTK-XNAT supports such an action...

I was able to allow folder creation for the scan folder via the GUI, but as you wrote, CTK-XNAT does not support such an action.
There seems to be a problem with the creation of a new folder: First a 'Resource' folder is created. After that the newly created folder is added to the 'Resource' folder. Unfortunately the creation of the 'Resource' folder is only allowed on the 'Subject' level and the 'Experiment' level.

On the 'Project' level the user is faced with a 'ContentOperationNotPermittedError - 202', stating "The operation request on the remote content is not permitted".

This leads to another bug, which is described here: T19905