I think it’s not a good approach to use Rasa as the centralized messaging router. It’s not designed to play this role. Even if you implement the suggested solution, your tracker store will contain conversations that are unrelated to the domain of the bot.
Instead, there should be a client UI that switches between Rasa and other services (Slack, another Rasa bot, etc). The action would send a message that the client UI recognizes as a transfer (via slot metadata) and pauses the conversation. The UI then connects to Slack or some other service.