When Bamboo is integrated with Crowd, Bamboo should allow going to the profile and changing the IM address and source repository alias of a user because the IM address and source repository alias is saved in Bamboo even though Bamboo uses Crowd as a user repository.
When Read-only External User Management flag is un-checked, and Crowd (I tried version 2.4.0) doesn't allow making changes for a specific user account, Bamboo shows this error if attempted to change the IM address (or other user settings).
Also, maybe the communication of Bamboo and Crowd can be improved to let Bamboo know about the Directory permissions in Crowd so that Bamboo knows that there can be no changes done (as it is read only). This way, let's say the Read-only External User Management flag automatically can be disabled if Crowd doesn't allow to make changes in user settings. However, this is not related to the IM address as it is saved in Bamboo only, and Bamboo should allow making IM address changes.
Check Read-only external user management at Bamboo Administration >> Overview >> Security settings