There's no simple script call to do it.
t = BlizzABS::Trigger.new
$game_actors[1].triggers.push(t)
# Testing if added
p $game_actors[1].triggers
This will add a new default trigger to Actor 1 (and print out the results that the trigger was, indeed, added).
You can assign values via the parameters when creating a new trigger.
t = BlizzABS::Trigger.new(1, 2, 0, 3, 1, 3, 0)
The order of the items is as follows: activator, condition, comparator, value, action type, action data, target.
The values you can use are
# trigger data
TRGLeader = 0
TRGAlly = 1
TRGSelf = 2
TRGEnemy = 3
TRGProbability = 4
# trigger target data
TRGTargetDefault = 0
TRGTargetActivator = 1
# trigger condition data
TRGHP = 0
TRGSP = 1
TRGState = 2
TRGLocation = 3
# trigger comparison data
TRGEqual = 0
TRGNotEqual = 1
TRGGreater = 2
TRGGreaterEqual = 3
TRGLess = 4
TRGLessEqual = 5
# trigger action data
TRGAttack = 0
TRGDefend = 1
TRGSkill = 2
TRGItem = 3
# trigger location data
TRGClosest = 0
TRGFarthest = 1
So in the above example, I made a trigger that activates by Ally (1) if the State (2) Equals (0) Venom (State ID 3). The actor will defend (1), so the 3 is useless since it's not a skill or item being used, on the default target (0).