List articles from current category AND subcategories?

Title really says it all. I need this data in the theme. How can I get it? I’ve found some very old forum posts here that apparently involved changing core files, which I can’t do (and it would likely break stuff and make updates hard, given those posts are several years old).

I hope, that you are able to read german. There is a module, that makes it possible to count and view the articles of all subcategories.
http://forum.oxid-esales.com/showthread.php?t=19732#post147800

I can read German, however I cannot figure out where to get the module. There is no “Count Article” Module in oxid exchange at least (search came up empty). And Google doesn’t lead to anything useful either.

[Edit]
I forgot to mention that I cannot buy anything. I have to make do with free solutions or base functionality (and I can’t do anything about it, as I’m not making the decisions).

just assign the articles of all subcategories to the parent category

I suppose that’s something we could do. Maybe there is even a module to automate the process? Again, a search came up with nothing.

it takes about 10 seconds and 4 mouse clicks per category to assign the articles.
Unless you have 1500 subcategories, it will take you longer to install some module and make it work.

Fact of the matter is that this may well happen. As far as I know, there are plans to connect the Shop to an ERP system. I have no idea how many (sub)categories there will be. But for now this will do, thank you very much.