You certainly can add some essential oils to your humidifier and the humidifier will disperse the essential oils into the air just fine.
But an essential oil diffuser is much better for dispersing the essential oils into the air.
The essential oil diffuser will nebulize the essential oils into miniscule particles which makes the essential oils easier to inhale when it's dispersed into the air.
So you'll get better health benefits when using a diffuser to disperse the essential oils into the air than you would with a humidifier but you certainly can use a humidifier with essential oils if you want too and get some of the health benefits.