Yes you can and its easy, for DRM-free ebooks, anyway.
With the optional GrayNM FBreader...
https://www.mobileread.com/forums/showthread.php?t=65865
...you just go to the settings page and choose "modify styles". There you locate body text and set it to left align. You can tweak pretty much every supported tag and define a global style for them that overrides the ebook's madanted formatting.
Its *your* reader and *your* book, right?
As a point of fact, I am currently reading a book in left-aligned, "ragged right" mode so I know it works.