I'm curious to know if it would be possible to give players the option to either set all doors (as the plugin stands now) or choose timers on a door-by-door occassion?

Perhaps something similar to this:
/ad all - Set the time before ALL of your doors automatically close (5-20).
/ad single - Set the time before the door you're looking at will automatically close (5-20).

OR:

/ad all - Set the time before ALL of your doors automatically close (5-20).
/ad door - Set the time before all Single & Double Doors automatically close (5-20).
/ad hatch - Set the time before all of your Ladder Hatches automatically close (5-20).
/ad window - Set the time before all of your Window Shutters automatically close (5-20).

Updated