Correct me if I am wrong, as far as I am aware, there is not such thing a graphene nano-tube. Graphene is called graphene because it a sheet, once you wrap the sheet in ca cylindrical (tube) shape it becomes a Carbon nanotube.
You can obtain some nanotube structures in .cif format following the steps bellow:
(1) Open the software VMD (Visual Molecular Dynamics), go to the option "Extensions > Modeling > Nanotube builder", and there you can build up some nanotube structures based on their chiral index n, m, and also controlling their length.
After that, you should go to "File > Save Coordinates...", and then you save the nanotube structure in .xyz format.
(2) Now, using other software called VESTA, you will be able to open the .xyz file of the nanotube and then save it in .cif using the option "File > Export Data...". I'm sending you an example of .cif fime obtained by this approach. Regards.