From the downloads page I got:

Qt 5.1.1 for Windows 64-bit (VS 2012, OpenGL, 522 MB)

In the CMake Wizard this gives me access to:

NMake Generator (Desktop Qt 5.1.1 MSVC2012 OpenGL 64bit)

which is terrific.

Now to my question. If I want to use the 32 bit version, do I have to download this:

Qt 5.1.1 for Windows 32-bit (VS 2010, OpenGL, 504 MB)

Or can I can add the 32 bit NMake Generator, to the Qt Creator install that I already have and if so, how can I do that?