There is no way to transfer the ER destinations from one legal entity to another and from one sandbox environment to another environment. It is time-consuming to maintain in different environments, especially if your client has a lot of legal entities and other sandboxes. We tried to look at a SQL script, but that was impossible.
We are doing it manually every time we have a database copy from PROD to another sandbox. Here we manually change/replace existing values, so the UAT environment doesn't trigger any Production documents by email. And we must repeat it in every legal entity, which makes it very extensive and time-consuming to maintain in different environments and entities.
As of now, there is no way to automate the transfer of the ER destinations. It could be great if there was an export/import function as well, just like the one we have on the Application-specific parameters.