You've actually got several companies selling suit systems that do just that for example, the Tigullio Defender All, which comprises a 5mm fullsuit, a 5mm shortie, a 3mm vest, a 5mm hood and 5mm boots. You can layer up in various ways as and when conditions dictate.
Use your shortie over the top of your full suit and it will give you more warmth around the important area your torso. A hood might also be a good idea.
Basically, if this keeps you warm, who cares what it looks like? Many a time I have been on assignment somewhere with my 3mm fullsuit, got chilled by the end of the week and stuck a 5mm shortie on over the top. Different make, different colour, but who cares I was warm!
Mark