Can I turn a Piezo Buzzer into an ultrasonic Mist Maker?
Piezo transducers for creating audible sound operate in a bending mode and resonate, typically, at 2kHz to 4kHz. 

Piezo tranducers used in nebulizers operate in a thickness mode and resonate, typically, at 1MHz to 2.5MHz (1.6MHz is a popular choice). 

They're quite different (1000:1 in resonant frequency)- just go and buy the right thing for the job.