You can't (as far as I know) specify the axis. That would require exporting to Blender and flipping it, which is probably the route you should take so you have fine grain control. Is this a static mesh or is it animated?
If you go that route it's best to use UModel for your mesh exporting. I've run into a ton of bugs using batchexport, WOTgreal, etc.
"You damn kids, back in my time we made the items, maps and games ourselves with an unwieldy engine using counter-intuitive crash-prone tools and we liked it so much we built communities around this which nowadays look like cults because they're quasi-parallel societies based on the same old games." -Hellkeeper