Do you have access to the file on the Chrome OS device?
The one called AquaMail.accounts.backup?
If so, just copy it using (anything really) to the phone, it should be inside "storage root" -> "backup" -> "AquaMail"
where "storage root" corresponds to the phone's large built-in memory partition, the directory where file manager apps open into at startup (most of them I suppose), the one that starts with "Android", "Alarms", "DCIM"...
Once that's done, AquaMail on the phone should be able to find the file in its Backup and Restore window.