Players poached from the PS have to remain on 53 man roster for first 3 weeks of season or cut.
In Corral's case, he was not poached from PS, but simply signed off of waivers.
If Patriots wanted to get Corral onto PS, they would have to first release him and then he would have to pass through waivers before Pats could sign him to PS.
So, if Pats want to keep him, for now he has to stay on 53 man roster or risk losing him to another team (unlikely, since no one else put in a claim for him)
I suspect....Pats keep Corral on 53 man roster but leave him inactive for many weeks until he has learned playbook sufficiently (maybe 6 weeks). Until then patriots can promote Zappe as backup QB to active roster for each game for 3 weeks (while leaving Corral on inactive game day roster) and then he immediately returns to PS after the game. After first 3 games if Corral is not ready to be backup they will have 2 options...either promote Zappe back to 53 man roster or start the 3 game promotion from backup squad all over again with Cunningham.