You can leverage the Set-Mailbox cmdlet and the -WindowsEmailAddress parameter for this:
Set-Mailbox room123 -WindowsEmailAddress ******@rooms.domain.co.uk
The above cmdlet will add ******@rooms.domain.co.uk alias and set it as primary, while preserving the previous primary SMTP as secondary one.
To do this in bulk for all rooms (remove -WhatIf once you are satisfied with the result):
Get-Mailbox -RecipientTypeDetails RoomMailbox | % { Set-Mailbox $_ -WindowsEmailAddress "$($_.Alias)@rooms.domain.co.uk" -WhatIf }