I will see if I can define what you are talking about, otherwise, post an example.
Phone A: x4000
Phone B: x4000
Phone C: x4000
Phone A, B, C all share extension x4000 as their primary line. On Unity Connection, x4000 is a mailbox for these (3) phones. If a message is left, it will light up the MWI and leave a envelop next to that line.
If you want to add subscribers into Unity without adding lines onto the phone, just add a new Subscriber and line. If it's a DID, you will need to add a CTI route Point so it routes it to Unity from the outside/inside world. If it's just for internal, use a Non-DID number within your dial plan.
The only problem, if Sally at Phone A with x4000 has another mailbox on Unity with x4500, she will not know if she has new voicemail unless she dials into to Unity and checks. Unity has no way of notifying Sally at Phone A there is a new message. (unless you use notifications or IMAP, etc) If you want MWI or envelop on the x4000 line, you will set this up, which could be confusing for the Phone A user Sally. Will the message be for x4000 or x4500?
Let us know if this helps.