Can I automatically add all users that join a supergroup into a channel?
I don't think there's an existing way of doing that, but maybe it can be achieved via the API.
Note: I heard of people that added multiple users to channels and then got banned/restricted in some way or another. I don't know (and couldn't find out) what are the limitations before it would happen.
I think that the most probable way is to use the Telegram API to achieve that, maybe by creating a little script/app that will read the users from your group and add them to the channel.
Have a look at the messages.addChatUser method.
A question on StackOverflow that asks how to do it with a bot:
If you don't want to risk anything (such as being restricted), I would go by engaging users to join by themselves:
- Create a nice post with an image an a button to the new related channel.
- Forward messages from the channel to the group.
- Pin an invitation message to the chat (assuming you're the admin).
Note that if you're not the admin, they will probably won't be happy with your advertising..