(Edit, I think my original conclusion here was wrong, having read up about it).
While it seems probable that Pluto and Charon were formed by collision, I gather it's unlikely that this collision was so recent as to explain their lack of craters and young surface. The Collision, by most articles I've read, happened when the solar system was young, not in the last 100 million years.
Now, it's possible that none of this is certain, but I retract that part of my original post.
But, the creation of Charon by giant collision remains likely See here, and here. That would suggest formation within our solar-system, not from outside.
Pluto/Charon are also among a fairly common orbital region called Trans-Neptunian objects.
According to Wikipedia they suggest the collision that formed Charon happened 4.5 billion years ago.
Sedna (love the story, by the way), has a very elongated and distant orbit which seems more likely with capture of an object. I agree with everything James Kilfinger said. Rogue planet/Rogue object capture may happen from time to time, but the Rogue would need to get quite close to the sun for the sun to have sufficient gravitation to achieve the capture. Highly elongated orbits would seem likely for solar-system orbital capture. Pluto is only slightly elongated and not a good candidate. More massive suns should have significantly greater capture numbers, but that's kinda obvious I suppose.