You don't want to split related files across drives. If you break up a multi-part rar archive you won't be able to extract files with winrar or play a movie without extracting it, etc. Since ioFTPD doesn't offer support for it natively you also won't find zipscripts that will either so it's just not going to happen.
There was recently a good topic/thread in this forum (
Merge Drives) where I explained how things work now and potential future features. Check it out as it contains some good examples.