You already have a CMS Geeklog 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 Geeklog and without give their personnal msn/yahoo email address.
They will only use their Geeklog identifier (nickname).

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

In IntraMessenger admin (ACP) select and valid option:

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

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

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.