You already have a forum FUDforum and want to add a desktop Instant Messenger ?
IntraMessenger is the (3rd-Party) add-on you looking for !

Users will be able to contact your community without need to be connected in FUDforum and without give their personnal msn/yahoo email address.
They will only use their FUDforum identifier (nickname).

How to do in FUDforum ?
NOTHING to change inside FUDforum (a good news !).

In IntraMessenger admin (ACP) select and valid option:

if necessary copy FUDforum paremeters (tables prefix, host, database, login and password) and determine (address) url to register to your FUDforum server.

On (Windows) clients, install and choose the url of installing IntraMessenger, just use login and password (same FUDforum).

Test the admin control panel live demonstration (ACP) !

External authentication documentation how to setup include in server.

Extern authentication example :

Its purpose isn't to be one more MSN/Yahoo/Jabber/skype/... like.
It's a PHP/MySQL server, allows flexibility, and simple and easy adaptability.