Yes, you can do that, just like adding support for say the PlayStation Move or 3D TVs. That doesn't equal a good experience though necessarily. So many game experiences suffer in VR if you're not designed for it at the start, and jutter/motion sickness is a real concern in VR.
VR generally wants you to have a slower paced first person view, if the player isn't in a cockpit of some sort and in a "natural sitting" position. Racing and flying games have this and are naturally good VR experiences if the engine will allow for it (not all do). But all 3rd person games are out. Most FPSes are out. Thats a good chunk of the industry.
VR definitely needs to be developed for intentionally. Not least because the performance requirements are significantly higher.
However VR isn't really restricted to a first person view, you definitely need to respect motion sickness and avoid it as much as possible but there are plenty of good games that use a third person viewpoint on the action. What you really shouldn't do is take control of the camera and when you do (e.g. for locomotion) avoid styles that cause nausea.
FPS games actually work great if you get the motion control scheme down. Initially FPS games didn't work well until people came up with good solutions to this.
VR generally wants you to have a slower paced first person view, if the player isn't in a cockpit of some sort and in a "natural sitting" position. Racing and flying games have this and are naturally good VR experiences if the engine will allow for it (not all do). But all 3rd person games are out. Most FPSes are out. Thats a good chunk of the industry.