I know there're powerful TFS command line tools from TFS Power tools that provide much more that can be achieved through Visual Studio integrated capabilities.
I have several changesets that aren't associated with any workitem. I would like to create a new workitem and associate these existing changesets with it.
Is this possible? I don't see any particular reason why not, but it depends whether command line tools provide such functionality.
Sure you can associate a work item to an existing changeset. You can do that directly in the Visual Studio IDE. Here's how:
First open the work item - and select the All Links tab:
Then click on Link to and select the changeset and click Ok: