Unfortunately, it is not possible at this moment to use list module to add other controls (such as PostsList) inside of the List Items.
However, you could achieve this by developing your own control to display lists (in other words your own version of PostsList control) where you'd use Sitefinity API to get the lists (in Telerik.Lists.ListManager class you have methods such as GetLists()), provide UI for your users to order / nest lists how they see it fit and then display it. The idea here would be to have a template for list (take a look at our templates in ~/Sitefinity/ControlTemplates/Lists) and then instantiate a list template inside another list template, in order to get a nested list effect. I am not saying this would be a trivial task, but it is possible.
Also, I'd like to say that your idea of having nested lists seems like a very useful one, so we'll discuss it and see if we can provide this functionality out of box in one of the future releases.
the Telerik team