You can do this with the "Archive Mail" feature in MailScanner.
1. Create a rules file in /usr/mailscanner/etc/rules. Call it, for example, archive.rules
2. In that file, add a line like the following examples for each domain/address that you want to copy all mails for:
# If you want to copy all mail going into a
# particular domain to a specific mailbox
To: *@domain1.com firstname.lastname@example.org
# If you want to copy all mail to or from a certain
# account to another mailbox
ToOrFrom: email@example.com firstname.lastname@example.org
Adjust as needed to accomplish what you want. There is no need to include a default line.
3. In WHM - ConfigServer MailScanner FE - MailScanner Configuration, search for "Archive Mail =". Enter the following value:
(or whatever you called your rules file)
Click Change to save the changes and restart MailScanner.
You should ensure that you are familiar with and comply with your country's privacy laws before setting up mail archiving.