If you have the Rocbox source (it must be a checkout newer than the revision where Antialiased fonts support was added, obviously) and you have compiled it at least once, I think you will find a command line tool named convttf will be on the "tools" folder of the source code.
For more information, see the log message of the revision in which AA fonts were introduced into the official builds:
http://svn.rockbox.org/viewvc.cgi?view=rev&revision=29523 (revision 29523)
In case you don't have the source and you don't want to build it, I don't know if there are any precompiled binaries around (but I don't think so).