1000v with 10mv resolution is 1/100000 so you need 17bit ADC that can give a resolution of 1/131071 but then your problem will be to use a divider to lower this voltage to a level that the ADC can work with (depends on the ADC ref voltage) and still be able to keep the noise in very low level to have a correct measurement, I have no idea how to do that.
Assume that you have 1000v input, to fir that in a 5v ADC you need to divide it with 200.
Now imagine that you have 0.1v and you divide it also with 200, the result will be 0.5mv so you can easily have noise problems.
Maybe you can do it with variable dividers depending on the range, I can't give you any recommendation because the voltage is too high and I have never done anything similar
Alex