Jager,
I fully understand the principle of the extra gear and in fact the toolbox came with a nylon/plastic 40T gear on a short spindle identical to the idler at the spindle. This may or may not have a purpose and Warco hav'nt a clue and say, when challenged, the lathe is'nt capable of left hand threadcutting.
For myself I am unable to understand how or where a second idler can be inserted.
I don't have your lathe, but I do have a lathe which just as in your description, uses an extra gear with the same number of teeth as the main driving gear on the rear of the mandrel. This extra gear is mounted on a short spindle to provide left hand gear cutting. This extra component fits into a purpose made slot on the rear mandrel bearing cover. It thus adds an extra gear into the chain and thus reverses the drive to the leadscrew.
Now I know this doesn't apply to your lathe, but I offer it as a thought as to how it might work.
FWIW, adding, or subtracting (if possible) just one gear into the gear train will reverse the leadscrew.
Also, perhaps having a look around the other importers to see if they say it can cut L.H. threads might be of use. Also, Little Machine Shop in good ol' USA might have some information on the internet.
Failing all that, how abour posting a photo of the gearbox then people can see what's what.
Peter G. Shaw