i should have given some more information about the existing plumbing. there is a half bath in the attic already(sink and toilet) the sink and toilet are connected to a 3" stack that goes to the downstairs bathroom and through the roof as well. thanks again
Do you have easy access to the plumbing fittings just below? I believe today's standards might require running your new shower drain all the way down to below your lower-floor fixtures, but I also have an old house and I would evaluate the possibility of connecting it right there under the floor.
That does not give much more enlightenment than the original posting. We would have to know a lot more about how the existing plumbing is installed before we could even guess as to the proper piping, and even then it might not be a DIY job. Call a plumber to look at what you have and discuss the feasibility for a shower with him.