EnableBridgeConferencing in the client options section of the Jabber config enables this option.
I'm not sure how this interacts with the Bridge URI dialing.
I was able to play with this a bit - you can actually set this up with Conference Now if you'd like although it is hilariously jank for this type of system.
Put the Conference Now number under where it says "Number". Then, put your meeting number under both the Host and Guest PIN. These labels have nothing to do with the called service, what it does is dial the number you put there, waits ~3 seconds, then dials the "Host PIN". At this point you'll have to open the keypad and enter your UCM PIN number as the focus seems to go back to the chat box right away. If you don't, you get put on hold as an attendee and you're stuck until it hangs up or you redial. If you hang up there's a "Join" button that appears so you can press that then try again.
Jabber prompts the other people you have escalated the call to if they'd like to join. It then dials that number from your client's parameters on their end, and enters the "Guest PIN" which again would be your meeting number. They can then press # or sit and wait and will get tossed into the conference.
The participant's list doesn't work anyways so this just looks like you and the conference number on the sidebar. But otherwise the apparent intended behaviour is that it starts a conference "window" and you can merge the contacts in.
Given that you can start a conference call from both a group chat and a buddy group that's probably safe.
