How can I append a different element for each list in a column in pandas?

I have a dataframe, df, with lists in a specific column, col_a. For example, consider that the first element is [1, 2, 3] and the second is [4, 5]. Please note that they can be lists of different lengths.

I want to use conditions on these lists and apply specific modifications, including appends. For example, imagine that if the length of the list is > 2, I want to append another element, that is the sum of the last two elements of the current list. So, considering the first list above, I have [1, 2, 3] and I want to have [1, 2, 3, 5].

What I tried to do was:

""" df.loc[:, col_a] = df[col_a].apply(lambda value: value.append(value[-2]+value[-1]) if len( value) > 1 else value) """

But the result in that column is "None" for all the elements of the column.

Can someone help me, please?

Thank you very much in advance.