Using your harmony would be very round about to say the least. I don't know which harmony you have, but they are all IR, even the rf ones use proprietary connections from remote to base station so the ir is all you could hope to use.
You could use a device to read the rf comming off of the harmony or its base station, inject that signal into a network enabled device which then sends a network command to the itach.
I have a harmony 890, ISY, and an itach wifi ir. I use my Harmony to send IR commands to my ISY which then executes various functions. I could set the ISY up to execute a network command to the itach and it would work. Currently the ISY does send IP commands to the itach, just not as a result of receiving anything from my Harmony.
For example, I have programs on the ISY that when I push a button on one of my Insteon keypad links, it tells the ISY to turn on my whole house music, turn on the satelite, and set the satelite to a specific channel, all by sending network commands to the itach.
My Harmony sends commands to the ISY via the ISY's IR input. For example, I can push a button on the Harmony and have various lights turn on, or it arms my Elk security panel.
In short, if you have an ISY and any IR device and an Itach, you can use the IR device to get the itach to send commands via the ISY. There are other devices besides an ISY that can do this. They would plug into your computer and then you would have an application on the computer that turns a received ir signal into an outbound network command.