Hi @SteveS,
No there's no option to do either of those at the moment.
There's a related feature suggestion here about channel admins - it would be great if you could upvote/add your feedback there, and maybe add a new one for inviting team members to all channels when they first sign up?
We definitely want to improve the onboarding experience so people can choose more default channels when users first sign up, so it's great to get more feedback about what people are looking for.