import numpy as np N = 10 a = np.random.rand(N,N) b = np.zeros((N,N+1)) b[:,:-1] = a
np.concatenate((np.zeros((3,3), dtype=int), arr), axis=1) array([[0, 0, 0, 1, 2, 3], [0, 0, 0, 4, 5, 6], [0, 0, 0, 7, 8, 9]])