Here is a tutorial on how to use PHP to write to files in the filesystem.
PHP Tutorial - File Write

If its a straighforward append to the end of the list, then that should do it, If the IP address has to go in a specific location in the file, it wil get a little more complex.

You will need to load the file into a variable then do a search and replace for the location it needs to go before writing it again.

if you have time try to learn and do it yourself it might take you a couple of days but keep an open mind to learn new things otherwise you could hire somebody.
I'm sure it'll cost you max 20-30$ ... unless you add more stuff to the task