Well, you can put an agent ID in the TAC/Ext field on a BUSY-IND station button, but it won't work; it will never light up when the agent is on a call.
If you need to monitor when the agent is active, etc., use a CMS report (real-time; group or split/skill).
If you need to call the agent directly, but don't know the extension of where they are sitting at, dial their agent ID instead.
Susan
"'I wish life was not so short,' he thought. 'Languages take such a time, and so do all the things one wants to know about.'"
- J. R. R. Tolkien, The Lost Road