Converting structure files can be done with babel, you can check out whether the gsas format is in the list:

https://openbabel.org/wiki/Babel

On the GSAS-II project window, the Export menu has

Phase as - Quick CIF

and

Powder data as - Data only CIF

If you want to combine those, you should be able to figure it out using a text editor.

I am also searching same for Carbon nanotube tube. you try fullproof. and plot it on mercury software for getting CIF file . it may be helpful.