how to batch add a membership level to large amount of users

I've imported 3400+ users from an old site. I need to give them all our lifetime membership level (no charge). I see I can go through page by page (About 50 at a time) to give them all the level they need. Anyway to give this to all of them at once? I can't seem to find out how to display more than 50 on a page to check off. Thanks!