you can
just enter the path usr/share/flipclock/theme i guess , not sure.
but it has been mentioned in that announce thread of flipclock widget.
there are two folders one is for theme .
open the folder and then right click on the theme you are using and click open then in the middle of the page you can find the find name and scale etc , just change the name of the default font to your font .
but only those fonts will work which are in your device
i know i dont have to mention this but i and we all know noobs



p.s only if you are using winscp to explore your phone :P
and plus that's the first thing i had done when i installed this widget and it worked for me