So you're thinking of giving the user electric shocks for a more immersive experience?

Kidding aside, could be interesting, but you'd really need to care about safety issues. Some of those stimulators (especially for muscle stimulation, I don't know if you're including those or not in your TENS definition) can be pretty dangerous if misused, for instance placing the electrodes at the wrong place...
Also, you'd have to make sure "wiring" the TENS as you said does not break its inherent safety. Putting anything between the stimulator unit and the electrodes could be a safety hazard.
Another related issue is that your system wouldn't allow to control any of the TENS parameters, which is a good thing per se, but also a bad one, because the settings (current, pulse width, frequency, etc) would completely depend on the user, and they may take risks they would not otherwise.
Sure if you're considering interfacing ready-made units, you may be thinking that all safety concerns are upon the TENS manufacturer's shoulders and not yours, but if you're inserting things before the electrodes, you're effectively modifying the device. Besides, if the intended use is recreational, that could mislead people in using the TENS device with much less precautions that they would otherwise do (again parameters-wise or relating to the electrodes' position)...
Just saying.