I've never heard of a gameport to serial
converter.
It is possible to connect a number of gamepads to a parallel port. (playstation, nintendo, sega etcetera)
There is a free windows driver and schematics
at this site
http://www.ziplabel.com/dpadpro/
There is also a dos TSR whose name escapes me at the moment.
(linux users will find support for parallel port gamepads built into the joystick driver.)
Chris