HELLO

I plan on install casement windows. We are considering building out the window slightly so we can have a larger sill without extending inward to the living space. I have notice homes the small style as mine that appear to have this done. Can this be done? Can I simply add 2x4's around the outside perimenter of the current jamb/header? The window (New construction) would then be installed on the new jamb. There is an overhang about the window @ 2ft deep, @8in above. Anything to worry about with such a install?