PDFs with embedded fonts should work.
The iLiad's support for adding new fonts is not very robust. The best thread is
Problem reading UTF8 Chinese HTML files, but this suggests several approaches. If your font file is small enough, the best approach seems to be to add it to /usr/share/fonts/truetype (see posts #18 and #19). If the font file is large, then you can create a fonts directory on your cf card and put it in there (then reboot). Note that this can hang your iLiad (see post #14). This should be enough to get the font into FBReader 0.8.14. There is an extra step for fonts on the cf card with FBReader 0.8.04 - let me know if you need this.
So far as I know, no one has looked in detail at the DR font setup. The actual setup is typically defined in /etc/fonts/fonts.conf. If it is configured like the iLiad (i.e. like most Linux systems) user-supplied fonts can go in the directory ~/.fonts and the text file ~/.fonts.conf would define where else they can go (e.g. on the SD card). These files do not seem to exist.