example.py

l = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]
a = ['a','b','c','d','e','f']

for step in range(1, 6):
    step_result = []
    letters = iter(cycle(a))
    for index, num in enumerate(l, 1):
        step_result.append(num)
        if not index % step:
            step_result.append(next(letters))
    print(step_result)

[0, 'a', 1, 'b', 2, 'c', 3, 'd', 4, 'e', 5, 'f', 6, 'a', 7, 'b', 8, 'c', 9, 'd', 10, 'e', 11, 'f', 12, 'a', 13, 'b', 14, 'c', 15, 'd', 16, 'e', 17, 'f', 18, 'a', 19, 'b', 20, 'c', 21, 'd', 22, 'e', 23, 'f', 24, 'a', 25, 'b', 26, 'c', 27, 'd', 28, 'e', 29, 'f', 30, 'a']
[0, 1, 'a', 2, 3, 'b', 4, 5, 'c', 6, 7, 'd', 8, 9, 'e', 10, 11, 'f', 12, 13, 'a', 14, 15, 'b', 16, 17, 'c', 18, 19, 'd', 20, 21, 'e', 22, 23, 'f', 24, 25, 'a', 26, 27, 'b', 28, 29, 'c', 30]
[0, 1, 2, 'a', 3, 4, 5, 'b', 6, 7, 8, 'c', 9, 10, 11, 'd', 12, 13, 14, 'e', 15, 16, 17, 'f', 18, 19, 20, 'a', 21, 22, 23, 'b', 24, 25, 26, 'c', 27, 28, 29, 'd', 30]
[0, 1, 2, 3, 'a', 4, 5, 6, 7, 'b', 8, 9, 10, 11, 'c', 12, 13, 14, 15, 'd', 16, 17, 18, 19, 'e', 20, 21, 22, 23, 'f', 24, 25, 26, 27, 'a', 28, 29, 30]
[0, 1, 2, 3, 4, 'a', 5, 6, 7, 8, 9, 'b', 10, 11, 12, 13, 14, 'c', 15, 16, 17, 18, 19, 'd', 20, 21, 22, 23, 24, 'e', 25, 26, 27, 28, 29, 'f', 30]