[spence88mph]

here is the solution given to me by xcart, very helpful and took no time:

You can use the 'group modify' feature to resolve this issue.

For this you should specify the category, the products of which you want to be displayed in the root category, in the "Search in category" field, tick the "Search-and-modify" check-box and after that click on the "Search"- button.

After that you will be redirected to the section of all products modification, where you can define the additional categories for the group of products.