This is expected and reasonable behavior.
For example, I have a live account named firstname.lastname@example.org and my email(also ocs) account is email@example.com. This is a conflict and communicator won’t understand who should be added.
The resolution right now is that MSN will periodically check the live account. If it detected someone’s live account has conflicted with MSN federated partner, this live account will receive a notification that he must change this live account to another email address.
During that period, we can contact with our internal OCS user only. Here are the steps to add like kind of contact. We should remove contact from our contact list. Type the name of user in communicator search bar. Add the user listed into our contacts list.
In this way, msn user won’t be listed and only internal OCS user only. So, we can send IM to this user.
Note: This is OCS communication. It is not possible to contact with MSN account anymore if his live account conflicted with MSN federated domain.