a.cumsum() + b
сделает это.
Например,
In [11]: a
Out[11]: array([3, 5, 1, 4, 2, 6, 0, 5])
In [12]: b
Out[12]: array([1, 2, 3, 4, 5, 6, 7, 8])
In [13]: a.cumsum() + b
Out[13]: array([ 4, 10, 12, 17, 20, 27, 28, 34])
Сравните это с:
In [14]: [a[:k+1].sum() + b[k] for k in range(len(a))]
Out[14]: [4, 10, 12, 17, 20, 27, 28, 34]