Short answer: you can't, don't bother.
Longer answer: You would need to defeat Secure Boot first. All RT devices have mandatory Secure Boot, and unless you could get Microsoft to sign your image, it wouldn't boot. Even if you could, though, there's very little chance of you getting it working. For one thing, even if WEC13 has the required drivers for *some* of the RT's hardware, it surely doesn't have all of them. For another, unless there's been a major change in the CE kernel recently, it doesn't support symmetric multiprocessing so you'd lose 3/4 of the CPU power of the Tegra 3.