1) Assuming you are making the command yourself and are running the bot as an admin, yes. With your command give the full path to where you need it to save to. For example for me to save the highlight text in my public dropbox folder it'd be
@savelog@[C:\Users\Jordan\Dropbox\Public\highlight.txt, @uptime@]
Using @savelog@ will make sure that you don't overwrite any previous highlights already made in there. This will of course mean you'll need to clear the list but otherwise, should work ok. Tested this myself and it does appear to work quite well.
To answer your original question though about being able to change the location of the highlight command itself, I'm afraid not to my knowledge
2) Chain another command (Under command chaining when you go to edit a command) on top of the custom one you make and use the following depending how you want it to be.
If you want the bot to whisper to the person
/w @user@ Highlight created, thank you
If you want the bot to just type in chat
@user@ just marked this down as the greatest moment in all history. Contribution to science recognised
Thanks,
Dante557