Co exsisting with FreePBX

While using FreePBX satisfactory, I need to setup some extensions and dial plans using the old .conf files
It’s mainly for the purpose of adding an AutoDiler capabilities.
If not for FreePBX I would use sip.conf and extentions.conf but in the case modifying is forbidden
So I have to use _custom.conf files But trying this was unsuccessful in my case.
I wonder if anyone had experience with it, possibly with an example.
Shimon Dekel