I think you are confusing neutral with ground. While both are grounded to the breaker box, neither a 10-50 nor 10-30 have grounds, only neutrals. You could do two things that are not up to code if you have a long wire run and for whatever reason dont run a new seperate ground wire to the 14-50 or 14-30. One is leave the ground unconnected, the other is hook the ground and neutral together(not recommended). If you leave the ground not connected to anything, you just have to remember to not plug in any device that requires both a neutral and a ground, or it may cause problems.