Can you put metal strings on a classical guitar?

Absolutely not.

Metal-string guitars are designed to sustain a much higher tension on the strings than their classical counterparts.

This is due to the difference in the materials the strings are made of. Nylon strings need a lower tension to work at the correct pitch, and as a result guitar bracing is usually not strong enough to work at the tension of metal strings.

Unless the guitar is equipped with truss rods to protect the integrity of the neck from sudden changes in voltage, metal strings should never be used in place of nylon.