You might think about the USB Interlink upgrade. You can hook your transmitter to that or use the interlink box itself. Even though you said not to use the RF mock transmitter, just thought maybe you didn't know about the USB upgrade?

There are some interfaces available that you could hook your transmitter up through the gameport. These are ok, but you are limited to 4 smooth channels, anything above would be a switch. So, for example, you wouldn't be able to do flaperons/spoilerons. They tend to be a bit particular about what operating system version you are running, so just ask the vendor before buying.

If you want more than 4 smooth channels and go with an alternate interface, be specific with the questions to the vendor about what it supports.