Once a search has been processed, it is possible to apply a tag to the results en masse. To do this, once the search results are displayed, select Tools at the top of the page and select Tag Documents:

This will bring up a window within which you can select the tags you wish to apply to the documents. Once the desired tags are select,  click Apply to apply the selected tag(s) to the documents listed in the search results.

Should the search results not quite reflect the documents you want to apply the tag(s) to, you can de-select documents via the tick boxes on the left of the page before applying the tag(s):