Is there a way to convert a stereo track into 2 mono tracks so that the left and right files are split onto the 2 mono channels? Or do I have to just export each audio file as split channels, then re-import?
Convert Tracks is a Nuendo 6 feature if I am not mistaking.
However, you can slit the files upon import.
On import of a file, you should get a pop-up screen with a set of import options.
One of them is split into mono tracks.
Fredo
Wasn’t this in N5.5? Hmmmm 
Could be right … Such a long time ago …
Fredo
Select the track, right-click on any event within this track, select “Convert Tracks …” from the context menu, choose the appropriate options,
… just verified in NU 5.5.6 Win7 64 bit. 
HTH,