Does anybody know how to (if it can be done) enable the doors to lock when you leave the car and forget to lock the doors? Any help would be appreciated.

From what I'm reading in the factory repair manual that's not an option. Here are the settings you can change (and how):

2. MANUAL SETTING (Switch operation)
(a) Close all the doors.
(b) Turn the ignition switch on (IG).
© Within ten seconds after the ignition switch on (IG).
• To set function (Shift-linked Automatic Door
Lock)
Push and hold the LOCK side of the power door
lock switch for five seconds with the shift lever in
the P.
• To set function (Speed-sensitive Automatic Door
Lock)
Push and hold the LOCK side of the power door
lock switch for five seconds with the shift lever in
any position except P.
• To set function (Shift-linked Automatic Door
Unlock)
Push and hold the UNLOCK side of the power
door lock switch for five seconds with the shift
lever in the P.
• To set function (Opening Driver's Door-linked
Automatic Door Unlock)
Push and hold the UNLOCK side of the power
door lock switch for five seconds with the shift
lever in any position except P.
If user wants to cancel a function, repeat the
procedure. Each time user perform the procedure,
the function is set or cancelled.

Maybe I misunderstood the OP's question, but wouldn't automatic locking be rather dangerous ....what if your keys are in the car????

Seems to me that this would be similiar to the automatic locking of the doors after the car starts moving. I would question the wisdom of making the car lock automatically when it's turned off.