Dear Danielle,

I have not the experience with plasma freezing, but I isolated total RNA from culture cells and blood. For both the most important is to freezing and storage the samples at -80 degree, because RNA is very unstable. In the case of blood we used the specialized kit for blood freezing, but I recommend carry out the isolation with unfreezing sample (as soon as possible).

Yes, you can add RNAlater. But remember RNA is easily degradable so it needs to be stored at -80 and keep it on ice during the experiment.