How to overwrite extensions_routing.conf at every boot of the system

As you told me before there is no way to write my own dial plans in extensions_routing.conf and this file will be removed on every boot of the system - this is not ok.
So how can I make a custom script or something else to overwrite extensions_routing.conf with my modification of the file on every boot of the system?

Hi, sir

you can modify the routing rules in web access.

Hello

There is a custom.sh under /etc/cfg/gw.

Ok, but how to use it? How to replace the existing file, just let me know the syntax how to do it with this custom file?