In Control under Content you are able to use the Move to button to move content from a group to another location such as a category or knowledge base.

However, when you have content within one of those categories or the knowledge base you cannot use the Move to to move content to a group. Instead you have to go to each individual topic (conversation or discussion) or article and select from the Forum location to add it to a group.

To me we should be more consistent in Control in the ability to move content between parts of the community.