Hi newbee here in a need of a decently powerful driver operating at ~3.6MHz. Was wondering if it would be possible to modify one of these cheap $10 China ZVS drivers to operated at such a high frequency.
First, as @Lightium point out HV is no joke. It really can kill you. Without experience keeping it safe, you shouldn't be working with it on your own. If you want to experiment with anything that exceeds mains voltages* you need to find a mentor who has the necessary experience so you can learn how not to die.
Second, on your specific question, not at chance unless you consider "modify" to mean replacing everything, including the PCB. Everything from the components to the PCB material and layout becomes critical at those frequencies.
the 30kHz rating is an order of magnitude too low
*Strictly speaking, I would actually say that anything above 50V needs experience and great care. That's the generally agreed threshold for inherently safe voltages. It's really hard to sufficiently express how dangerous things get as you increase the voltage and how unexpected the behavior can be if you are not familiar with it. Please take this seriously.
The resonant frequency is determined by the parallel resonant tank circuit formed by the capacitor and the magnetizing inductance of the transformer. You should learn about these and about HIGH VOLTAGE SAFETY!