imap access from zimbra

[kathmandu]

As far as i know we can not use zimbra to existing imap server and retrieve mails in zimbra mail interface. Is it true? we are planning to use zimbra for calendaring plus the web interface for existing imap server. Clients can add external imap accounts by themself but it would be nice if that can be done while creating accounts in zimbra but this requires password of existing users. In addition I am using external LDAP for authentication. I am lost in all these stuffs. Somebody help me.

Thanks in adv
Paras.