The official rule is:
Dreadball Rulebook Page 28 wrote:
Simply choose as many Coaching Dice as you want from those available to you, and add them to the hand of dice before you roll any original emphasis
It's a little ambiguous because it says choose as many coaching dice as you want but the before you roll in emphasized. Personally I would say that as long as the dice haven't been rolled then you can choose to add more in, even in response to you saying you were going to use one in defence. After all they wanted to use coaching dice to make sure they had one more than you and it may well benefit you further down the line because they've used up all their coaching dice.