#!/usr/bin/awk -f

BEGIN {
   if ( from == "" || file == "" || to == "" ) {
      print "Usage: replace-text FROM_PATTERN TEXT_FILE TO_PATTERN"
      exit
   }
}

$0 ~ from {
   system("cat " file)

   while ( getline ) {
      if ( $0 ~ to )
         break
   }
}

{ print }