There are some Action cards that affect (if boons trigger that effect) some more other targets than that against which Action Card was used..

My question is: should that add-targets use any Defence against that attack? Or only the main target can use defence?

For example: some Monster Action Cards state "if one boon is rolled this attack affects one more target engaged with Monster"... ok: that one more target can Dodge (or Block, Parry, etc) that attack?

If answer is "Yes", well, how can he do? The dice have already rolled, so how can he add Misfortune Dice to dice pool (already rolled)?

You answer yourself the question : you cannot. The attack is already rolled and is a success (you cannot apply that boon line if the action is not successfull i'd say, like a +1 damage or a +1 Critical boon effects)

The side effect (bonus) of the Boons is that a second target is hit too, even if it wasn't aimed at. No new check or Defence needed. You could say it's a surprise attack-type (side effect of the original action)