Welcome to TBN!!! I have seen one log splitter that mounts to the loader on a tractor. I can't remember the brand, but I think people said it would not be practical due to the price. You might want to ask in the attachments section.
Unless you have a larger tractor, the splitter will probabally be slower than you would want. Stand alone units are very well built and priced reasonabally. IMO, that is the way to go for splitting firewood.