You want to create an empty list, then append the created list to it. This will give you the list of lists. Example:
>>> l = []
>>> l.append([1,2,3])
>>> l.append([4,5,6])
>>> l
[[1, 2, 3], [4, 5, 6]]
Use append method, eg:
lst = []
line = np.genfromtxt(‘temp.txt’, usecols=3, dtype=[(‘floatname’,’float’)], skip_header=1)
lst.append(line)