There's no way to do this out of the box. However, you can set CB's setting for new user redirection to be their profile, at which time they could edit those fields. If they're marked as required, they wouldn't be able to save. While they could still navigate away from this page, making it not perfect, it's the best 'quick' solution I could give. I think there's CB addons out there that can check if a profile is complete and redirect a user to it constantly until it is. I say think cause I know there exists extensions like this for JomSocial, and I thought I've seen them for CB, but don't know of them off-hand.