From the CH knowledge base . . . . .
A gameport to USB adapter may work, but is not guaranteed. You will most likley come across a few incompatabilities, and maybe a need to calibrate more often than not. Also, the long term effects of the adapter are unknown, and could void your warranty. Your best bet would be to start off with a USB device instead of adding an adapter.
If you do encounter problems, you will need to contact the adapter's manufacturer for more information as this setup is unsupported by CH Products.
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Some have switches to support mutiple modes for different types of devices ilke the Rockfire does....other adapters will only support a generic 4 axis - 4 button config.