Contrary to popular belief several non T-Top cars, referred to as hard top cars, were used in the production of Knight Rider. The opinions of why this vary, but the production team would use hard tops for more severe stunts.
It is commonly known that the 82-92 F-Bodies were structurally the weakest GM ever made which is why subframe connectors and other body strengthening components are available. The example I remember most clearly is a guy parking one front left wheel on the kerb and not being able to open his door; such is the movement in the frame!
Here is an excerpt from the knight rider historians website :
Starting in roughly the second season, a sharp eyed viewer could begin to spot non T-top versions of K.I.T.T. on the screen. The producers were very clever in their attempts to make the cars look like they had t-tops: they applied trip pieces where the t-tops seams would be. Also, non-top cars received a boxy overhead console as opposed to the form fitting one seen in the t-top version. Check out some examples below, including an interior shot of a hardtop car.
And here are some photos from the same source :
I also managed to find photos of a guy who has modified his hard top to resemble a T-Top from the inside AND the outside which will probably be similar to the path I will take.