The preparation of TNT from toluene involves nitration (by electrophilic substitution) of an aromatic molecule. Fullerene is not aromatic, and cannot be nitrated by the same mechanism as toluene. It is possible to produce nitro compounds by other mechanisms (e.g. in the preparation of nitroglycerin), and it’s even possible to nitrate alkenes. Fullerene would function as an alkene for the purposes of nitration, and it is actually possible to nitrate fullerene (apparently reaction with nitrogen dioxide is one method). A fullerene with six nitro groups is mentioned here.
So, it is possible to make nitrated fullerenes. Whether such a compound would be a practical explosive is a rather different matter. To be an effective explosive, a compound must oxidize rapidly and must produce a large number of gaseous molecules (e.g. nitrogen, carbon dioxide) in its decomposition. TNT is a powerful explosive because its decomposition produces a substantial amount of nitrogen, carbon dioxide and water vapor. A fullerene with six nitro groups would not produce substantially more gas upon oxidation than a fullerene with none. Enthalpy would also need to be considered; fullerene, being a product of combustion itself, cannot be expected to release a very large amount of energy upon decomposition, and I doubt a hexanitrofullerene would release all that much more. A more extensively nitrated fullerene might indeed be a very powerful explosive, but it would also be wildly unstable, probably sufficiently unstable to prevent its manufacture, let alone safe handling. Ring strain and strong repulsive interactions between adjacent nitro groups would probably either make the reaction stop or break the molecule apart well before you made an explosive better than TNT.