My team are on task to create full-automatic uav to fly to distination, collect packages,and RTB. For safty,we decide make the aircraft disarm itself when arrives at distination. When the packages are loaded ,the crews at the distinationw will push a button on the aircraft.It will arm and start the RTB flight plan.
Can we program it to arm or disarm itself?
Thank you. 
on a multicopter, when land it will disarm by itself after some second thanks to parameter http://ardupilot.org/copter/docs/parameters.html#disarm-delay-disarm-delay
There is no easy way to allow the drone to arm itself, for safety concern of course. But in your case, you will have an operator asking to fly back with button so that will be doable.
With a suitable onboard companion computer yes. Look at the documentation.
http://ardupilot.org/dev/docs/companion-computers.html