"Do you add ..."
Not usually. There are a few PSU's that you can do that with. On most PSU labels there's a second line that will tell you the total power you can pull of the 12 volt rail.
One way to get around that is to use a single rail PSU such as Corsair or PC P&C. The single rail technically violates the ATX specification but Corsair (and I, having two Corsair PSU's) do not worry about it.