I currently have a downstairs toilet only, the waste outlet on this goes directly into the floor.
I need to fit an upstairs toilet. Can I plumb the waste outlet into the current stack pipe which services waste from the bath & sink?

It sounds like you would like to drain the upstairs toilet directly into the vertical vent pipe. That will not work well. If you are going to pull a permit the inspector will usually help you plan what you are doing or at least he will tell you if your plans are right or not. So, the best thing for you to do is get a diagram of what you want to do and alter it so that it reflects your own project and then ask the inspector if he will pass those plans. I will save you some time and give you something to work on or at least give you an idea for you to consider. Check this page out. http://www.plumbinglessons.com/2/2toilets.JPG
If the image comes up small just roll your cursor over the page rather slow until the enlarge arrows appear in the lower right hand corner of the page.