I want to use tracks to control the harmonies, but my product doesn't have an Aux IN. Is it still possible?
Yes, if your product has an INST IN/THRU (Harmony G-XT comes to mind)
You'll need a few adapters/cables, but you can definitely do it. The trick is to plug a "cable to nowhere" into the INST THRU to disable any onboard guitar effects and prevent sounds coming from the main outs of the unit. That way, the music is "listened to" by the system, but not heard through the device.
Connect your TC-H unit and the mp3 to the PA the way you normally do.
Here's a sample connection diagram: