In order to edit that file you must stop Kodi first. When it loads the file on startup it will remove any path it doesn’t like. I had this issue with it not wanting to let me set an artist information folder. I went round and round till I found the way to set a custom media source and then I was able to get it to go. More specifically, I could get it to take if I included credentials, but I really didn’t want to do that. If you wanted to you would add the source as smb://username:password@ipaddress/yada/yada/. If you want it to do it a better way try what I outlined in this other thread and then just browse to it from the gui in the normal way.
Or you should be able to use system mounts to work around the issue as well.