Hi!
What you need cannot be done with a simple telegram bot, but I have a solution!
That's what I would do: an automated Telegram client (that simulates a human user) reads the messages from the selected channels. The client passes the messages to a telegram bot, which will act as channel administrator and forward any message to your channel.
Sadly, that's not a trivial task, that's the reason behind my offer.
Anyway, I can do it, I can do it well and I can do it fast.
If you have any question, don't hesitate to ask!
Looking forward to hearing from you,
regards,
Davide