When does SCOTUS decide to hear new cases?

Hey everyone. I’m wondering when the SC decides to add a new case? As I understand, they’ve agreed to 50 for the year and have scheduled 30something.

My question is, when and how does this work? Can they decide to add a new case to their docket that previously hadn’t been included in the 50? Do they pick them all at the start of term and only hear those they initially committed to?

DACA is making its way through Texas courts and will eventually end up at the SCOTUS. If they wrap up in April, I’m wondering how soon SCOTUS will add the case to their docket. Thanks!