Yes it can be done. I just converted my 91' LX hatch to GT style with all the GT stuff from my wrecked 90' GT. Yours would just be the reverse of what I just did. You would just have to do the following:
1) remove front and rear bumper covers, install LX bumper covers.
2) remove side GT ground effects, drill out rivets that hold the ground effect mounting strips on.
3) drill out rivets for the mounting strips that mount the GT fender extentions on the front and rear.
4) remove door moldings (easier said than done)
5) install LX molding on doors, front fenders, and quarter panels (5 per side). The ones on the door and behind door install using double-sided tape, the ones on the fenders and on the quarter panels install with nuts/screws from behind. You may have to go ahead and install the front fender extentions also, as the Saleen air dam may need them there to install properly... not sure about that.
6) then, since you would be going with the Saleen style, you could just leave the holes that the GT mounting strips were mounted in and install all the new ground effects on top of everything, since that's how they're installed anyway.
One important note... when searching for the LX moldings, make sure you get the correct ones for the front fenders for your particular year. the 87'-90' ones are different than the 91'-93' ones due to the introduction of the pony rims in 91', which were larger than the wheels from previous years. The only thing that is different is the fenders, the fender moldings (front side only) and the fender extensions.
Hope this helps and let me know if you have any other questions.... by the way, I have all my old LX stuff in the garage that I will sell if you're interested. Bumpers, moldings, everything!
Good luck with it!