Hi Jocelyn,
Here is the info on how to allow Meetings to be extended. My understanding of this feature is that it is automatic. Have a look;
Extend meeting (minutes)
Whether or not to extend meetings if they run over the requested duration and if ports are available. Meetings may continue to be extended, as long as ports are available, up to the Maximum meeting length (minutes) field setting.
If you select Yes, then also enter the number of minutes to extend meetings.
If you select No, or if ports are not available at the end of the meeting, then callers receive a warning that the meeting will end. The warning time is determined by the Last warning time field.
Restriction: This field does not apply to reservationless meetings.
Yes/No
Range: 10 to 60
Default: 15
From this Admin doc;
http://www.cisco.com/en/US/products/ps6533/products_configuration_guide_chapter09186a0080788064.html#wp1018630
Check out Page 148 of this good User Guide for reasons when a Meeting will be Automatically Extended;
http://www.cisco.com/application/pdf/en/us/guest/products/ps6533/c1626/ccmigration_09186a0080788fc9.pdf
Hope this helps!
Rob