But that's only a part of the office contents. Even in the case of file cabinets, the lead vapors and the microscopic iron spheres would be carried away with the smoke, leaving only the largest iron spheres.
The possibilities of what nano-thermite can do are not know by the public. This thread is about establishing temperatures far in excess of what office fires can attain. So far, you have not provided a viable alternative to the extreme temperatures of 2800oF and 3100oF it takes to melt iron and vaporize lead.