How add MinGW build setup to already installed MSVC setup?

I have downloaded and installed the Qt 5.1.1 Win 32-bit MSVC 2010 pre-built binary including Qt Creator. Now, I’d like to add the MinGW build chain as a kit to Qt Creator. What is the best way of just adding this into the existing installation? I tried downloading the MingW prebuilt package and pointed the installer to the same directory where I had installed the MSVC package but it complains that Qt is already installed there and to choose a different directory. But if I do, I will get Qt Creator and everything else installed twice, which is not what I need.