I have Traffic X, but it contributes very few GA aircraft to  my FSX Australian airports, and I'd like to add more. I also run  the HTAI  package of AI Cessnas , which works fine in Fs9 - but I wonder if it can it be adapted to FSX, and if so, how?  FSX has empty Aircraft and AI folders, so I'd imagine there's a way ...advice would be very welcome.

As mentioned above, I would suggest downloading the World of AI GA packages - I think they're up to 7 now?

If you're interested in bringing back the fsx GA and not Orbit or World Travel, you can always download http://www.flightsim2004-fanatics.com/uploads/DefaultGA/Def_GA_DL.htm the fsx GA only files. 

By Traffic X are you referring to UT?  Or Traffic 2007?  Is Traffic X an fsx compatible only program? or is it also compatible with fs9 AI?

If you want the GA packages listed above to be 100% "fsx" compatible you'll need AIFPC to convert the Ttools flight plans.