Forfeiting action and additional attacks

I had a rules discussion last night pertaining to forfeiting a model's action and the ability to still make additional attacks during that model's activation. This pertains to models that CAN make additional attacks after their initial attacks (warcasters/locks and warjacks/beasts buying additional attacks with focus/fury, usually). The answer is no, additional attacks cannot be made, since if the model chooses to forfeit its action (i.e. combat action = initial attacks) because of some effect, ...