Numpy: How to append 2d array to the end of 3d array?

I create a masked array A with shape (8,1,51,120) and a data masked array B with shape (11232,51,120).

Now I make a loop to select bar of B[step,:,:] array and append it at the end of A[foo,0,:,:]. Here is some pseudo-code to describe what I want:

for step in range(B.shape[0]):
    foo = select_func(step)
    A[foo,A.shape[1]-1,:,:].append(B[step,:,:])

print(A[foo,:,:,:].shape)

Finally I want A[foo,:,:,:].shape = (bar,51,120).

It turns out very hard to append B to A using np.ma.dstack( or np.ma.append( not only because A not change as I do but also it made new masked array.

I try this: Append 2D array to 3D array, extending third dimension, but I want my original A array grow up not to make new array.