Slow cooker rump roast

You mean stew it in the soup? Nothing to stop you, I guess, but it's going to be a pretty big piece of meat to stew, the outside may be falling apart before it's done- you'll end up with vegetable beef soup and a piece of meat that will probably want saucing. Or you could braise it- preferably in the oven for a piece that size; rump roast is the standard meat for pot roasts in Portugal.