Well, actually it is possible. It is quite a bit tricky, though.
You need to make sure that for the certificate you create you can export the private key -- this option is disabled by default. If you create such a certificate, you can export it from one computer and import it on another and ioFTPD will work with it.
I have done it once, but it took me quite a bit of time to figure it all out -- in other words, try every other option first
