The steps to my front porch are exposed. The treated and stained handrails have started to rot (one out of 4 rails at this time). I am not prepared to replace all the porch and stair railings right now. My question is whether or not it is possible to replace the 4 exposed top railings with Trex railings, attaching them to existing wooden balusters.
There is no reason you cannot mix Trex type boards/railings with wood products. Decks that use Trex type surfaces are usually framed with pressure treated lumber. If your balusters and other parts are not either pressure treated or regularly stained/sealed/painted, they will also deteriorate fairly quickly.
If you go ahead with Trex on wood, be sure to use coated screws meant for composites.
These have a special finish that resists corrosion and a special thread that grips composites well.
Some people use stainless steel, but I have heard reports of staining despite their "stainless" characteristics.