He proposed that only those countries which were willing and able should opt in to phase three. [VERB PREP. + to]

他提议只有那些有意愿而且有能力的国家才应参与第三阶段。

He didn't exactly opt out because he never opted in. [VERB PREP.]

确切地说他并没有退出，因为他压根儿就没有参加。