Here We can do it per Extension :
Yes, but that is getting way beyond Elastix and into raw Asterisk programming.
What was said above was reference of how to handle it inside the GUI itself.
Both methods work, but dropping to manual editing of configuration files should normally be avoided whenever possible as that makes it harder to use the built in tools for backup and recovery.
Also, upgrades will often over write manual changes like this, meaning you have to remember and reply them after updates/upgrades.