Это то, что вы хотите?
>>> x = np.arange(12).reshape((4, 3))
>>> x
array([[ 0, 1, 2],
[ 3, 4, 5],
[ 6, 7, 8],
[ 9, 10, 11]])
>>> shape = x.shape
>>> np.append(x, np.zeros(x.shape[1])).reshape(shape[0] + 1, shape[1])
array([[ 0., 1., 2.],
[ 3., 4., 5.],
[ 6., 7., 8.],
[ 9., 10., 11.],
[ 0., 0., 0.]])
>>>