Hi Manu,
Sure, this is most possible :) Outside callers can attend the Conference via a DID (Direct-In-Dial) Meet-me number (if configured) or via Transfer in from the main Operator.
Here is some additional info;
Initiating a Meet-Me Conference Bridge
Meet-me conferences require that a range of directory numbers be allocated for exclusive use of the conference. When a meet-me conference is set up, the conference controller chooses a directory number and advertises it to members of the group. The users call the directory number to join the conference. Anyone who calls the directory number while the conference is active joins the conference. (This situation applies only when the maximum number of participants that is specified for that conference type has not been exceeded and when sufficient streams are available on the conference device.)
When you initiate a meet-me conference by pressing Meet-Me on the phone, Cisco CallManager considers you the conference controller. The conference controller provides the directory number for the conference to all attendees, who can then dial that directory number to join the conference. If other participants in a meet-me conference press Meet-Me and the same directory number for the conference bridge, the Cisco CallManager ignores the signals.
The conference controller chooses a directory number from the range that is specified for the conference device. The Cisco CallManager Administrator provides the meet-me conference directory number range to users, so they can access the feature.
Conference Bridge Configuration
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00801ecf8a.html
Adding a Meet-Me Number/Pattern
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00801ecf96.html#76011
Hope this helps!
Rob
