If your ePub is DRM free, you can load it int he Calibre editor and edit the CSS and change where it shows...
Code:
text-align: justify;
to
But before you do that, do you really want to do that because of some study? There are studies that say this or that makes reading easier and they are rubbish. For example, there's a thread on here about a font that's said to make it easier and faster to read and that is rubbish. So don't take all studies as true. Try left justify and try full justify (with hyphenation) and see what you feel works best for you.