Unfortunately, most migration options available are to help migrate accounts TO Gmail rather than from it. Migrating email accounts can be a bit messy in any case, but I’ve found a few links that should help.
First, to export your contacts:
You may be able to use Google’s Mail Fetcher here to easily share messages and data between multiple email accounts:
There’s a good selection of third-party apps specifically to help with account migration, and you should be able to find something here to accomplish the task: https://www.google.com/enterprise/marketplace/search?query=email+migration
Alternatively, if they use mail clients already to send/receive mail, all old messages can be archived/downloaded via the mail client. Please see: