We have a screen door on our Lexi 2012 Lexi 283ts, and it is curved slightly at the top to fit the contour of the door. However, when I took a good look at it, it appears to be part of the complete door/screen door/frame assembly. On the bottom of the door itself, next to one of the hinge's is a sticker with Challanger Door on it, probably the manf.
Don't know if your older unit came with one originally, but it dosen't look like you could retrofit one without taking the whole assembly out.
Just for the heck of it, I just googled Challanger RV Doors, and their web site has rv doors both with and without screen doors. You may want to contact them and see if the one you have can be retro-fitted with a screen door.