Can I turn off the N6030A arbitrary waveform generator predistortion from the N7620B Signal Studio for Pulse Building to get more power?

No. The N7620A software does not allow the N6030A predistortion to be turned off. Instead, it rescales the pre-distorted waveform to ensure that it is using the full range of the DAC. The N6030A predistortion is very good at correcting the Arb output.