Actually , it's already a condition, they just have not bothered to check on them. “The IRS has the same requirement for contractors, but they are monitored with much less frequency.” Of course, they blame that on their tight budget, but since it can be done with a computer check, I'm having a hard time buying it.
Under "client communications" there is a "preferences" section where you check off the ---engagement letter---. Then it should show as addable, just like the organizers.