How to harden brass

For ordinary yellow brass, no. The only way to harden it is by work-hardening.
Some other copper alloys can be hardened by a variety of heat-treating methods. Beryllium copper can be age-hardened like heat-treatable aluminum grades. Some obscure copper alloys that contain nickel or chromium can be hardened by equally obscure heat-treating methods (transformation hardening; heat-soaked phase transformation).
This is not hobbyist stuff, for the most part, but you can handle beryllium-copper age-hardening if you have some good temperature control at around 700 deg. C.