It is not allowed under Support Files - Language Independent. However you can add folders under Support Files - Advanced Files - Other. The documentation snippet is:
To add a file or folder to a disk image folder:
[LIST=1]
Click the Other node in the Support Files view. The Files pane is displayed on the right.
Right-click anywhere in the Files pane and select Insert Files or Insert Folder, or place your cursor in the Files pane and press INSERT (to insert a file) or Ctrl+Insert (to insert a folder). The Open dialog is displayed.
Select a file or folder in the Open dialog and click OK to add the file to Last Disk.
To specify the disk, run the Release Wizard and in the General Options panel, click the Other Disk Files button.
You can find the whole document and the references by looking up 'Adding Files and Folders to the Other Disk Folder' in the help.
