Stephen Fitz-Gerald
Ah,this picture makes your challenge a whole lot clearer.
I suggest you'll have to make a die for your press big enough to squash the whole spiral pattern to the proper radius of the spiral stair.
As you said,the good news is it's a repeating pattern,the bad news is you'll probably have to have the female piece of the die"bumped" at some industrial facility out of 1/2 inch plate.
Probably about $200 to have that outsourced.You can make your own male part for the die.Even a flat plate will conform to the lower die shape after the first press.
How strong is you press?