A simple script like this will do the trick:
transmission = nlread("file.nc",TransmissionSpectrum)[0]
transmission.current(spin=Spin.Up)
transmission.current(spin=Spin.Down)
Copy/paste this into the Editor, replace "file.nc" with the real name (incl. path) of your NC file containing the transmission spectrum, and send to the Job Manager, and the spin currents will be printed in the log window.
If the NC file contains more than one transmission spectrum, you can use
transmission = nlread("file.nc", object_id="gID002")[0]
to select the precise one you want (check the object IDs in the Result Browser by selecting the NC file in the main VNL window).