This article provides a good overview of the process with pictures: https://spinbackup.com/blog/how-to-import-mbox-to-gmail/

To complete this process, you will need:

• the password to your personal mailbox (@gmail.com, etc.)
• the password to the @brown.edu mailbox
• a standalone (free) mail managing software, such as Thunderbird: https://www.thunderbird.net/en-US/
  • the "ImportExportTools NG" add-on to Thunderbird

Once ready, the following steps will need to be taken to move mail into an @brown.edu account.

1. Obtain a .mbox file of the mail you want to transfer. When using an @gmail.com account, the easiest way to do this is by going to https://takeout.google.com and following the steps to export an archive of the files from your @gmail.com account. Those files will export as .zip files, which can then be opened to find the .mbox (mail) file.
2. Open Thunderbird and add your @brown.edu email account to the system. It will ask for the password once in a normal text box, then again in a Google pop-up box.
3. Create a new folder within your @brown.edu mailbox to hold the imported mail
4. Under the Tools menu, select the ImportExportTools NG option to import the .mbox file
5. Select the .mbox file you extracted from the old personal email account
6. Wait for the transfer to complete. It is best if you have a solid internet with good upload speeds and avoid the Brown VPN while completing this process.

When that process is done, the old mail will be in your @brown.edu account and accessible for use.