I wore a 7mm jacket over a 7mm fullsuit once, and while very warm it's not something I ever tried again. There's just way too much bulk on the arms--mobility is awful, and fitting compasses, computers, etc. around the wrists becomes difficult. For shore diving, I felt like I was gonna die from overexertion before getting into the water.
A much better solution would be to buy a 5mm hooded vest, or a short-john, to wear underneath your full-suit. Having 10mm+ of neoprene on the core isn't too debilitating like it is when on the arms and legs. But if you don't want to spend money then can always just give the jacket a try.