To perform DOA estimation, the data input has to be across number of channels, i.e., number of elements in the array. On the other hand, in theory you can generate a range-Doppler map for each element for muleiple sweeps. Therefore, as long as you use the same operation to generate the range-Doppler map, you can try to reshape the map into a vector and then pass the vectors from all elements into DOA. This being said, you could also just pass the original received siganl across the array to DOA, there is no reason you have to do range-Doppler response first.
HTH
