I understand that you want to change the disarm signal for PWM outputs in the motor block. It appears that the disarm PWM signal cannot be changed from the block in the support package. As mentioned in the help document of the block, to arm or disarm the PWM output, you need to feed boolean False or True values to the 'ARM Output' port.
In order to modify the disarmed PWM output, you may need to open and modify the 'sfun_px4_pwm.tlc' file. This file is in the '..\pixhawk\blocks' folder of the support package.