Yes, you can apply paint over stain. Given that stains are generally applied in thin films, and it may be well-weathered at least in some areas, we encourage you to apply an exterior acrylic latex stain-blocking primer first. Then apply a top-of-the-line 100% acrylic latex paint in a flat or satin finish. In preparing the surface, go over all the old stain with a stiff wire brush. If you have cedar shakes, use a hand brush, using vertical-downward strokes. Then clean off the surface, and apply the primer.
Here are some additional tips: The extremes in temperature and sunshine should be taken into account when applying primers and paints. Care must be taken that they can form a good film. This means they should not be applied at a temperature lower than the recommended minimum temperature for the product (typically, 50 degrees F.)... this includes temperature of the surface being painted; and application should not be done if the temperature is predicted to be below the recommended minimum in the next 36 hours. Also, don't apply paints under conditions that will make them dry very quickly: do not paint in direct sunlight, and in warm, dry, breezy conditions.