if you read the manual and do this within Vicidial, you'll be able to upgrade/update later without breaking it.
if you modify extensions.conf directly and later upgrade ... your changes may be lost OR your newly upgraded system may not work as expected (because of an outdated extensions.conf file with your changes still in it) OR you'll have to make all your changes again.
have you read the manager's manual? It's available for free at EFLO.net (there is also a paid version that will likely save you several hours of learning, and considering the value of this software vs its cost, it's a worthy investment). I've had MANY customers pay me to fix things that would have been easily resolved with the manual.
(Not that I prefer you to give money to The Vicidial Group instead of me ... but at LEAST read the Free One!)