There are plenty of ways how you can import pictures (like PNG graphics) to SketchUp, you can use and import them as objects, use them for textures and probably plenty of more, so some kind of png to skp conversion should be doable.
You can download the basic version of SketchUp for free, but for more functions you would need to purchase commercial (Pro) version. Some other programs might also support SketchUp formats, but using the original is always better choice.