Block IMAP External Account but still available

[shouy]

Hi!
This is my problem :
I have to add an external email account in IMAP in order to access the vocal messaging system of the enterprise (a mail is sent with the vocal message in attachment). I've created the IMAP acount for an user and all is OK. But, now, I would like to block external account adding without blocking the IMAP account which is already created.
In the COS, when I modify the IMAP external account parameter, it delete all IMAP accounts already added.
The goal of my problem is that the administrator can add the IMAP account and then block the IMAP external account adding to users, but this account must still be available.
I don't know if it' possible to make this !

Thanks in advance for your questions and answers.

Steven