I see. If I understand well, you are saying:
Let’s say you have a category ABC and you posted in it during a year, let’s say 2005.
Now you want to create 12 subcategories in ABC – one for each month of the year. It is possible, but you will have to go manually through every posts and add the new (sub)category to it.
But for 2 years you’ll end up with 24 buttons in your navigation!
Also, please, try to remember what I already said several times: you do NOT put posts in a “normal” or “abnormal” page (Page?). You cannot put them anywhere. Posts go where they belong.
Also, as a general note. Instead of struggling to make WP to do things that is not meant to… it’s always better to understand how WP works and use its flexible features. Don’t go “against” WP – try to use it to your advantage. Trust me, it’s easier. It requires a learning curve but at the end it is rewarding!