Toping off carboy

Would it hurt/help to top off the carboy that I have my cider fermenting in? There is about 1/3 to 1/2 gal worth of room left to get it up to neck level.

Thanks,
Dano

How long has it been fermenting?

If you add fresh juice after a couple of days you’re adding O2 which could lead to oxidized flavors in the finished cider.  If it’s been less than 24-36 hours it will probably be ok, but after that I wouldn’t bother.  It’s only 1/3 to 1/2 gallon, I don’t think it’s worth the risk.

Well, it has been 3 weeks so I will just leave it alone.

Thank you!
Dano