I have an old T1942 Tacx trainer that has a wired USB connection.  To make it work, I wrote some code to convert the measurements (wattage, speed and cadence) from the Tacx Fortius to ANT+ format and transmit them out an ANT+ stick.    It compiles and runs in Linux but you can run it in a VM or on a $10 Raspberry Pi Zero.

So, you will need an extra ANT+ dongle (for a total of two) and either a VM (Virtualbox or VMWare player running Linux) or a Raspberry Pi. 

https://github.com/pepelkod/AntBridge

I plan to add a Raspberry Pi image complete with compiled code and auto start in the near future.