Yes, Font Collector (
www.zlthemes.com/Programs.php) will convert from: Fonts4OS5, FontSubst, iSilo, Plucker (non-aa), PalmBible+ (non-aa), VersaMail, Fonthack123, FontHackV, fonts-built-in-apps (any apps!) and built-in OS fonts, to: Fonts4OS5, FontsSubst, iSilo, Plucker, Palmbible+, Fonthack123 (including eReader) and Mobipocket. Conversion to iSilo, Plucker and PalmBIble+ is completely free. Otherwise, shareware $5 (or you can endure the converted font having a dot over every 's' character!)
Conversion from Fonts4OS5 or FontSubst to a different format is only supported for registered Fonts4OS5 or FontSubst users (respectively).
Allows vertical size decrease. (Make the fonts tighter vertically.)
Works on OS 3.5 and up, except for conversion to Fonts4OS5, FontSubst and FontHackV which requires OS 5 just like these programs do. (Conversion FROM Fonts4OS5 should work on OS 3.5 (untested so far), as long as you install (but do not run, since you can't run it) a registered copy of Fonts4OS5 in RAM.)
Supports Hi-Res+ and the like on every extended display I know of. (Sony VG, Handera VG, Palm DIA, Tapwave, Dana Alphasmart.) (Exception: Sony OS4 units have a lo-res font adjustment display, though everything else is hi-res. Sorry--I don't know a way around that right now.)