Can I set arbitrary SPI clock speeds?
You are limited to what your microcontroller supports. You could use a 12MHz oscillator to clock your Arduino and set the SCK divider to fOSC/2 = 6MHz (even if it was possible, I wouldn't go higher. Having some margin just to be on the safe side is generally a good advice).
Drawback of this solution is that you lose CPU performance. Please consider also that just collecting ADC data is not enough. You also have to do something with it. Therefore the question should rather be:
"What minimum ADC acquisition rate do I need for my application?"