I've never tried to reinforce pants. I'd think you'd need to do it just right or you'll get uncomfortable seams that could cause chaffing.
I know from experience, some fabrics are simply tougher than others. I've had certain types of sweat pants that hold up great, while others begin to quickly pill up and show signs of wear. It's not necessarily the type of material as it is the tightness of the weave and other factors. I have a pair of Nike golf shorts that show absolutely no signs of wear even after years of using them on the bicycle. I also own some pants from Banana Republic that have shown zero issues from using them on the bike.
The reality though is I ride wearing lycra bike shorts on the majority of my rides and they last for years.
Rather than try to reinforce your pants, I'd recommend investigating different brands paying close attention to the density of the weave and the surface of the fabric. Or wear lycra shorts when you ride and change into your pants when you get to your destination.