To hold a servo motor at constant torque, all you need to do is provide a constant voltage. You can accomplish this using the analog output (DAC) of a FlexMotion board. To gain access to the DAC, configure the axis as an open loop stepper, then use the Load DAC.flx VI to output an analog voltage. The downside to this is that you won't have feedback, so you'll have no way to determine your position.
Thank you for your response. If I send a constant voltage, how would i know what the corresponding torque value is. The relationship between voltage and torque includes angular velocity and according to your response, i would not be able to read the angular velocity. Basically, i am trying to send a torque value (rather than a voltage value) and make the motor rotate at it. Thank you for your time.