Well...how much time are you willing to spend extending naps?
When Tyler was that age, I'd quietly go in his room a few min before he woke up (it was always 25-30 min on the dot). I'd gently put my hand on his chest, thigh, wherever and rest it there. I kept it there while he stirred and started to wake. If he opened his eyes, I'd use my free hand to cover his eyes (he was easily stimulated by seeing things, so once he started looking around, it was all over). I'd stay there, not moving, until he'd drifted back to sleep. If he woke up crying, I'd do pat/shh while shielding his eyes. It worked most of the time, and eventually his naps got longer (for the time being, anyway). I ended up spending most of his naptimes in his room. It was worth it, but very time consuming!