There is no direct way to do import subscribers from another service. Facebook doesn't allow it yet but there is a quick trick to do it:

Deactivate Manychat for your page
Sign up to Botletter
Connect your page to Botletter
Invite everyone that currently has an open conversation with your Page in the Growth Tool section:

That way, the Botletter opt-in will be sent to all your past subscribers. They will have to resubscribe but usually, at least 80% of people do it.
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