Raiden userfile converter.
Online, simple and easy.
http://mouton.abuserz.com/r2i/
Input both your default.user and default.allow (or whatever) files and hop, u get a packed .rar containing your ioftpd groupfiles, userfiles and \etc\passwd and \etc\group files.
Note: Since passwords can't be converted, all users have "ioFTPD" as password after the conversion. Be sure they change it quickly!
If you wanna check out the source, see
http://mouton.abuserz.com/r2i/code.php
Enjoy.