import numpy as np
xyz = np.array([[1, 2, 100],[2, 5, 100]])
resultcolumn1 = (xyz[:,0]+xyz[:,1])*xyz[:,2]
>>array([300, 700])
>>type 'numpy.ndarray'>

print np.concatenate((xyz, resultcolumn1.T), axis=1)

resultcolumn2=np.array([[300, 700]])
>>>array([[300, 700]])
>>><type 'numpy.ndarray'>
np.concatenate((xyz, resultcolumn2.T), axis=1)
>>>[[  1   2 100 300]
    [  2   5 100 700]]