Hi Essaki,

I believe the feature you are looking for is called Barge/cBarge

Barge adds a user to a call that is in progress. Pressing a softkey automatically adds the user (initiator) to the shared-line call (target), and the users currently on the call receive a tone (if configured). Barge supports built-in conference and shared conference bridges.

The Single Button Barge/cBarge feature allows the user to simply press the shared-line button to be added to the call. The Single Button Barge/cBarge feature supports built-in conferences and shared conference bridges.

http://www.ciscosystems.com/en/US/docs/voice_ip_comm/cucmbe/admin/6_1_1/ccmfeat/fsbarge.html

Cheers!

Rob