Thanks for your reply. I uploaded a .csv file with x,y,z columns for each station - which I inputted into the Update Elevation tool to create a new 'bathymetry' layer which contained a column of elevation values which appeared as points on the correct XScutlines (the XsecID and Stations columns null values only). When I imported the subsequent .sdf file, the XSs had no elevation/station data attached (presumably because the .sdf file contained no xsSurface layer).
Alternatively, I imported a .csv with x,y,z data directly to HEC-RAS, which projected the XSs with correct station/elevation values. The downside of this is that it does not assign extra useful data that RiverGIS does (e.g. reach length, topology etc).
I'm sure the process is much simplier than it appears, but I am beginning to think it may be faster to manually copy and paste the z values(!)