The Admin interface for Google G Suite for Nonprofits offers a Data Migration tool to assist you in transferring email messages from your old IMail account to your new Google G Suite account.
Domain Administrators for Parishes and organizations who are using Google G Suite for Nonprofits have access to this tool via https://admin.google.com
On Page 1, enter the source (Other IMAP server), the migration type (Email), the connection protocol (IMAP), and the server name (imail.goarch.org). The last section asks for an account on that server to prove that you have access to at least one account on it. It doesn't matter which account you use on this page as long as it is an account on the IMail server.
On Page 2, indicate the time span for the migration. By default it is set to migrate the past 1 year's worth of emails.
You can run the migration several times using increasing lengths for the migration, starting with 1 month, then increasing to 6 months, 1 year, 10 years, etc. to ensure the most recent emails are migrated right away. As expected, the shorter period of time you select, the quicker the migration should go.
The Data Migration tool will ensure there are no duplicates if you run it multiple times.
On Page 3, add the users you want to migrate. You can use either "Add User" or "Bulk migrate with csv". Either way, you need to know the username and password for all of the source (old) accounts and connect each with its destination (new) account.
After the migration has completed, you can exit the migration and start over with a new time span.
For questions and answers about the Data Migration tool, please see the following link
Google G Suite Data Migration FAQ: https://support.google.com/a/answer/7032598?hl=en