Yes, it is possible, but you probably need leverage (i.e. a competing job offer). There is some risk here: all negotiation works on the basis that if you don't get what you want, you'll walk away, and rejecting an offer after you've signed is pretty bad. They could rescind your offer if you don't approach this conversation delicately enough, and if you plan to stay in defense for your whole career, it is a smaller world than you think. That said if the economics of the offer don't work out for you, the best thing you can do is tell them quickly (bad news doesn't get better with age) and be honest that you didn't quite think this through. A good hiring manager will want to help solve that problem.