You need to use Microsoft Excel:
- Open Excel and start a new Sheet
- Go to the Team Tab
- Click New List
- Select your Team Project
- Select your Work Item Query
- If Tags are not part of your results:
- Click on Choose Columns from the Ribbon Bar
- Add the Tags column
Now just enter into the tags column in excel the tags you would like as a comma separated list.
Then just click "Publish" to push it back to TFS.