It is possible to replace the font used by the MobiPocket Reader, see
Mobipocket default font?. I don't know if using (say) the Liberation Font would add Greek to MOBI or not. You should definitely reboot after adding the font.
It is also possible to use other fonts for FBReader, but it can be tricky to install them. This may not be necessary because DejaVu Sans is already installed and supports many languages. Are you using FBReader 0.8.14 (full screen, but with a toolbar at the bottom of the screen) or FBReader 0.8.4 (with icons at the top of the screen)?