Howto add multiple extensions

i installed FreePbx and i need to add a lot of extensions (i used before not asterisk based PBX). One of the way is to change extensions_addtional.conf, but notes of FreePbx say ‘DO NOT EDIT THIS FILE’. If someone can tell me precise steps how to do it in a right way.

Download and in Module Admin add this module.

I add this module, and just now making some tool to generate csv i need. Thanks a lot for your help. You resolved my problem.