it seems only rectangle is available as a sheet at the moment by using command line?
It is possible to specify both parts and sheets in the CSV command-line integration and these can also be from CAD file.
Please see the "Integrating with External Systems" section of the help documentation (press F1 key with NestFab open). Then look at "Defining a CSV File of Part Data" (see the "IsSheet" column parameter).
Please let us know if you need more information. We are here to help.