Me again. No-one aswered to this question, and I thought it's just a site bug. However, just today, almost 4 months later, I accidentaly found the solution - in other words, yes we can add tags to old posts too! I'll post screen captures here, in case someone else has been wondering about this, too.
1. You can't add tags to old post within the editing view. (The editor box that opens when you click edit any particular blog post of yours). You will need to be either at the main page of your blog where all your blog posts are scrollable (example from my own blog: http://forums.longhaircommunity.com/blog.php?u=11704), or at the view where you have clicked open one particular blog post (but don't click edit, just the normal view. Example of my blog: http://forums.longhaircommunity.com/entry.php?b=120776.)
2. At the bottom of your blog post there are some information, like when your post was edited, tags and categories.
Note that this is my older post and I have not added tags when I posted it.
3. Now bring your cursor over or next to the text "Tags: none" (Of course this works for posts on which you already have added some tags, which you might want to edit, remove or add new ones to.). A symbol appears (see screen capture below):
4. Click the symbol, and a box opens, on which you can add new tags (or remove and edit old ones). When done, remember to click "save changes".
Done!
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