what size is your carboy?? Too much headspace is supposedly conducive to promoting the likelihood of infection.
Also, I'm not sure whether or not you could get away with diluting to the proper volume with, either, bottled water, or water you've boiled and then cooled to the same temp as your beer, because I don't know if it would be possible to get the two different gravity liquids to mix evenly without risking oxygenating your brew.
Maybe possible to rack to your primary vessel onto the 6 litres of water and then rack the mix back into the carboy without doing too much damage.
Disclaimer: This is purely conjecture on my part and not something I have practical experience of.