How to convert a chocolate fudge frosting recipe to white chocolate?
I'm afraid this is not possible at all. Dark chocolate flavor comes from aromatics in the cocoa bean, which give flavor already in low amounts. The flavor of white chocolate is very subtle, it's a combination of the aroma of the cocoa fat and the concentrated dairy solids. You cannot add it to anything as a flavoring, because practically anything else will have a stronger flavor of its own and mask the white chocolate taste. 

The only way to have a cake which is covered in something which tastes like white chocolate is to make a white chocolate ganache. But then the texture will be a ganache and not a frosting. You will not be able to use your original recipe. 

See this question for the correct ratio on white ganache, many people just try it like dark (that's what I did the first time too) and it ends up liquid.