If you don’t mind fading to black and then fading back in from black, you could probably use the Matinee functionality.
If you specifically want a cross dissolve between two images, like you describe… you’re likely going to need to do something like create a SceneCapture2D (and a render target), capture with a precise match to the screen resolution, and use the resulting texture as a material in a post-process layer. And even then, you might still get some artifacts, as it’s very tricky to ensure the texture you render into is identical. A quick search of the forums turns up a few examples of this technique, includingthis thread.
If you don’t require animation from the source camera during the blend, it might be feasible to take a screenshot into memory of the player view before starting the blend, then overlay the screenshot on the viewport, transition to the target camera behind the scenes, and then fade out the overlaid screenshot. This is not a technique I’ve tried, though, so I won’t swear it’ll work.