I want to cook it in my slow cooker. How long do you recomend? Should I add more liquid since it will be cooking longer?

I agree with Carla. no added liquid and the timing seems right on. Be sure your beef is browned prior to placing in the slow cooker. Let us know how it goes!

I wouldn't add more liquid. In a slow cooker, the moisture (steam) doesn't get a chance to escape like it does on a stove-top pot; slow cookers are built this way in order to make it possible to cook things for a long time without the worry of drying and eventually burning the contents. If anything, you might want to reduce the liquid a bit in order to better replicate stove-top results. My personal rule of thumb for slow cooker cooking is if something is only going to cook for 4 hours, use high heat; if something is going to cook for 5+ hours, use low heat. Good cooking and good eating!