how do I backup my other email accounts to iCloud?

You can add iCloud mail addresses in Mac system preferences > internet accounts .

See the articles Use your Internet accounts on Mac - Apple SupportUse your Internet accounts on Mac – Apple Support (AU)

Change Internet Accounts preferences on Mac - Apple Supportsupport.apple.com › en-in › guide › mac-help › mac

Add an internet account - Apple Supportsupport.apple.com › guide › mac-help › mac

The mails from server will sync and can be viewed in mail application Read emails in Mail on Mac - Apple Support

You can add company mail address see the articles Use Custom Email Domain with iCloud Mail - Apple Support

Add a custom email domain

To use a custom domain with iCloud Mail, you need:

• An Apple ID with two-factor authentication turned on.
• iCloud Mail set up on your devices.

To add a personalized email address for a custom domain that you use with iCloud Mail:

• That email address can't be used as the email address for a different Apple ID. 
• If you want to add a personalized email address that is used for a different Apple ID, you need to change that Apple ID email address.
• If you delete that Apple ID, you still won't be able to add your personalized email address.

Learn how to add a custom domain for iCloud Mail on iCloud.com.

You can add any mail accounts that you wish to mail on your Mac.

How do retrieve my old iCloud account