Memphis370Z

I thought it did this by default. At least mine does.

EDIT: Just found the following in the FSM. The service manuals can be found here, by the way - (Nissan 370Z Owners Manual / Service Manuals - Nissan 370Z Forum)

AUTOMATIC DOOR LOCKS
. All doors will be locked automatically when
the vehicle speed reaches 15 MPH (24
km/h).
. All the doors will unlock automatically when
the ignition is placed in the OFF position.
The automatic lock and unlock functions
can be deactivated or activated independently
of each other. To deactivate or activate
the automatic door lock or unlock system,
perform the following procedure:
1. Close all doors.
2. Place the ignition switch in the ON position.
3. Do one of the following within 20 seconds of
performing step 2:
. To change AUTO UNLOCK settings:
push and hold the power door lock
switch to the UNLOCK position for
more than 5 seconds.
. To change AUTO LOCK settings: push
and hold the power door lock switch to
the LOCK position for more than 5
seconds.
4. When activated, the hazard indicator will
flash twice. When deactivated, the hazard
indicator will flash once.
5. The ignition must be placed in the OFF and
ON position again between each setting
change.