"ʻAe, e pāʻina ana kākou ma Genki Sushi."
Translation:Yes, we are going to eat at Genki Sushi.
It should be accepted but they probably didn't think of adding contractions as possible answers.
So, is there a relation to the imperative construction? Is it a sort of a subjunctive?
E ... ana just shows incomplete action, not subjunctive. So here it is used for future tense. It can also be past imperfect as well.