Yes, you can use retroactively-added memories! So long as your character would've experienced the memories before coming to Beacon, you're welcome to use 'em. On a sort of related note, headcanon memories are also fine.

In the case of canon material released within (roughly) one month of the event start, we just ask that you spoiler-warn accordingly. We do have a rule that players can't app from new canon material until one month after that material's release, but since this isn't apps and we can't exactly say "oh, just memshare next month!", we're fine with new material so long as players is respectful about it. We'll make a note about that on the OOC info post as well so it doesn't get missed!