Zero Padding in Frequency Domain (Spectrum): How to Split the Middle Bin (Nyquist Frequency)?
One potential issue here is that you are technically in violation of the sampling theorem. Having non-trivial energy at Nyquist is typically an indication that the signal wasn't sufficiently low pass filtered before being sampled. So chances are you already have aliasing. If the Nyquist energy is low enough, it doesn't matter how you split it. 

Zero padding past Nyquist to upsample introduces a frequency domain "brick wall" filter between Nyquist and Nyquist + 1. You can certainly go through with the mathematical procedure but I would not expect this to closely match the behavior of the original signal.