HELP! Porblem with Alert info

I migrate to freepbx from simple asterisk. and now have problem with polycom IP4000 phones, when I dial to them I must send Alert Info to phones with description “Ring Answer” with asterisk I make it with code “exten => 100,n,SIPAddHeader(Alert-Info:Ring Answer)”. Can I do this with web interface? because when I make some changes with number from web program regenerate extensions.conf and deleting all I do

Edit /etc/asterisk/extensions_custom.conf

Any file with the word custom in it, you are allowed to edit and it will not be deleted.