Actually the analog joysticks are the very ones a generic gameport to USB
adaptor works with. The adaptor has circuitry in it so that the computer
recognizes the adaptor as a generic controller.
A force feedback controler probably doesn't use the analog capabilities of
the gameport though. It's probably using the gameport's less known serial
Musical Instrument Data Interface (MIDI). Even some of the non-force
feedback Sidewinder joysticks were "digital" joysticks that used the MIDI
capabilities. I've not seen any adaptor that claims to translate the MIDI
signals to USB. If there are such adaptors, they would probably cost as much
as a sound card anyway.
Controlers that use the MIDI capabilities ussualy woun't work without
Windows running, even through a gameport.