add 2nd icloud-account to mail (not alias)

Sorry to ask again, couldn't continue this one How to add/create a second personal iClou… - Apple Community

my daugther has an icloud-account, which for some very special reason we'd like to abandon, except for her email.

So I set up a new appleID / icloud.com-account, a new macOS user profile, but I do not arrive to integrate the old icloud account into native macOS ventura mail. i do not want to use 3rd party mail application, which seems like a workaround.

Same problem will be on the iPhone

iMac 2017/macOS 13.2.1 (which btw runs fine on big SATA SSD (no fusion drive))