Hello,
is it possible to add comments to a whole album, like it is possible to add a comment to a single photo?

Yes, of course. Some themes already have this. For others you would have to add the code. The current development stream has greatly reduced the difficulty in doing this, so you may wish to try the nightly build.

With the nightly build, all distributed themes except stopdesign (a difficult theme to modify) directly have the option of album comments. For any other theme you would need to modify the theme to use the new comment_form plugin.

Great, thank you!