Hello Chao,
Depending on your time frame, we have a new module to control Dynamixel X-series servo motors nearing release. I anticipate it becoming available before December.
If 100ms latency and jitter is OK for the door open/close commands, you can order Pololu Maestro for use with a standard hobby servo. Bpod includes a class to control the Maestro, and the state machine can trigger it via USB using soft codes.
If you need the precision of a stepper motor, you can have and Arduino control a stepper motor driver (e.g. this) via the Bpod Arduino Shield. This solution would require firmware development and more involved wiring.
Please let me know what you decide to do!
Thanks,
Josh