Simply place the filter capacitor in parallel with the lower leg of the resistive divider. High frequencies will view the capacitor as a short-circuit and bypass the lower resistor to ground. Low frequencies will view the capacitor as an open circuit and proceed through the resistive divider as normal. In other words, the resistive divider becomes a resistive divider where the lower "resistor" (which is really a resistor and capacitor in parallel) will see an impedance which varies with frequency, thus the ratio will vary with frequency in that the higher the frequency, the more the voltage will be divided by all the way down to zero, ideally.
You will have to adjust all the values from what you initially have though to maintain a particular divider ratio and cutoff.
You may want to buffer the the entire circuit before it enters the ADC with an op-amp voltage follower though.