You need to Transfer your domain 'for Hosting only' (if that's still possible?) , see the guide here https://www.plus.net/help/archive/broadband/how-to-transfer-a-domain/ . Your domain should then appear in the control panel and mail will be hosted by PN automatically , any mailboxes you already have ( or create in future ) will be applicable to both your <username>.plus.com address and your domain address.
Unfortunately the domain transfer system is an old system and needs manual intervention by a few knowledgeable staff. Certainly the domain transfer out automation is broken at the moment, but I'm not sure about the transfer in. I suspect it always was a manual system in that it raised a 'ticket', so it's probably still ok...
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
