Basically, you will have to build a staircase without treads, just risers.
Then you will have to bolster and heavily frame it from the outside.
There is a link in the helpful links to get you started figuring out how to lay it out and get started.
http://www.blocklayer.com/Stairs/StairsEng.aspx
Here it is again, It's worth looking at just to get an idea of the math involved. Print it and take it outside to the site and start planning.
Have fun.