Sounds like you are looking for a MeetMe number. It is the closest thing to what you are looking for without having to buy another product.
The MeetMe number must first be initiated by hitting the "MeetMe" softkey and entering the MeetMe number. Then, all attendees may join by simply dialing the MeetMe number.
Elliott
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/8_5_1/ccmcfg/bccm-851-cm/b03meet.html