Radius support isn't built into ProFTPD, though there's nothing stopping someone writing a module and submitting it for inclusion in the code tree. Possibly the easist way to implement Radius is by using the modules available for PAM and using the inbuilt PAM support.
This is possible using either PAM or the Opie modules. The module passes back a challenge which the user puts into a key generator along with their 'pass phrase' and it gives them back 5 words which get sent as the password. As long as you do it correctly it will never repeat.
It requires opie to be installed on the server. There are key gen clients for win95/98, *nix, mac. ftp://ftp.urbanrage.com/pub/c/mod_opie.c