As of last year, you would still have to take an EMT course. WI does not have a special certification for pre-hospital RNs as IL and several other places do. Though it might sound unfair, there are some convincing arguments for keeping it this way.
EMT tasks in the field involve more than just the medical side of things. You have to learn extrication techniques, crowd control, traffic control, how to provide treatment in an overturned vehicle and many other care and assessment practices that an RN isn't likely to have been taught. An RN taking an EMT course would no doubt have a leg up on students who have no medical background, but should not assume that they'll do equally well in the non-medical aspects. I have been in classes and on departments with farmers, engineers, teachers, and railroad workers who were aces at the technical aspects of climbing and lifting and extrication and was glad to have their form of expertise.
I would encourage you to take an EMT course for another reason as well.
While RN and EMT training contain a fair amount of overlap, there are some important distinctions between the two that can get you into a world of trouble if you blur the lines. EMT protocols are different. They focus on aspects of care that hospital folk rarely see. And they often rely on a person's skill in improvising and braving field conditions that you would not run across in an ER. It's vital that folks with previous medical training of any sort learn to put that background on a back burner long enough to acquire the EMT perspective or they will find it difficult to distinguish between their varied roles. This can mean overstepping EMT boundaries, an action that can create all sorts of problems.
Often, you can get permission from medical control to go above and beyond EMT limitations if you are an RN or some equally skilled type of clinician, but it's crucial that you are able to keep your focus consistent with the license you are operating under at the time. The skills are one thing. The legalities can be another.
If you are thinking of running with a particular department, they will usually pay for your training and perhaps let you go on runs as a trainee. This is a great way to get to know your future crewmates and learn where things are in the ambo and other fun stuff.
EMT training takes one semester. Yeah, time is tight during that stretch, but it's a wonderful way to grow into the new discipline.
I wish you well.