You can upload email files from a cloud server or files on your device to an email account.

Firstly open the Files screen inside AltaMail in the left side panel. Open the cloud server or the local files screen to view the email files. Tap on the file checkbox to select one or more emails files. Then select "Export to email account" in the action menu. (circle with 3 dots button)
Then you can choose an email account and email folder to send the email files to. This will create new emails with the contents of the email file.

There are various ways to save emails as files. Save as email file or Backup to email files
You can also move emails between email accounts without the need for saving emails as files. Move to email account