The CHARMM force field is designed for discrete small molecules, either as single residues, or polymers of small residues with a small number of well defined linkages. While fullerenes might be reasonable to attempt, large sheets like graphene or nanotubes with variable registration of the how the tube is sealed pose challenges for tools designed for residue based biological force fields. As it stands, you would first need to create a residue that defines the complete fullerene or nanotube, which can be used to create a PSF.
Since you've used Avogadro, one thing you might try is export the structure as a .mol2 file and then use the
cgenff tool to create the residue and assign the atom parameter types and bond based parameters. As usual, no warranty expressed or implied.
There are some (older) discussions of nanotubes in the parameter forum.
The
charmm-gui site has some facilities for building nanotubes.
