well you can do it in a number of ways:
1. Put in a value in the AXIS_SHIFT parameter of the NCSequence. + is away from spindle. This will offset your entire toolpath by this amount. Of course your first slice will remove more material but in non steel or a small offfset it shouldnt be a problem.
This would be the easiest method.
2. Create a Mill surface that simply goes deeper than your actual part wall.
This would be the best and most controlled method.
3. Perhaps, I havent tried this, you can specify the OVERTRAVEL parameter and get the desired result.
And that would be the guess method....
Good luck, Jonas