I agree a hooded vest may be a better option. It is likely you are not really getting cold on your arms and especially your legs which are moving most of the time (unless you are just drifting or hovering). I found that by just adding 2 to 5 mm to my torso area and effectively sealing the neck hole against outside water going directly to my skin (hood connected to vest), my whole body feels warmer. Add to that good gloves and boots to seal the other 4 holes in a similar manner and you can really improve the feel of you existing suit. Also, the vest covers the area of your back where cold water comes in through the zipper right to your skin. A skin will not really do any of that.
I have had good luck with a 2mm hooded vest either under or over a 5mm or 7mm full suit. I have also recently tried a 5mm hooded shorty over a 7mm suit. This really does a nice job of sealing the neck and zipper leakage and still leaves arm and leg movement uninhibited.