If this is a B&W slide consisting of a silver image, and not something dye based - then yes, you can intensify the image. But note that only image density that's already there, can be intensified. You would use something like a chromium or permanganate intensifier. The bad news is that even with repeated treatment, if the image is very weak to begin with, it's often difficult to make much headway.
For the most part, I'd consider an overexposed slide to be FUBAR.