The trunk will attempt to lock itself if you open and close the driver's door. This is a safety feature. It will do the same if you open the trunk by pressing the trunk release button. However...If you unlock the other doors and then get out of the driver's seat, the trunk will not lock itself!

I have looked extensively into retrofitting the soft close feature. It is not possible. The vehicle's 'General module' needs to be replaced along with lots of wiring and trunk motor, etc. The general module controls everything from unlocking the doors to vehicle sensors...not something you want to mess with or you will spend months trying to reprogram everything!