Yes you can. Just select the files that you want to keyword and type in the keywords that are relevant to all of the selected files. If you're a Power plan member you can even use the LightRocket tagging Thesaurus when keywording batches of files (usage is limited to 5 times a day with Launchpad and Portfolio accounts).
So for example if you had some images from a fashion show, you could select them all and add: Fashion show, Catwalk, Fashion, Models, etc. Then you could select only the ones showing clothes from different designers and add Gucci etc and tag/keyword those separately.
If you select files that contain different tags/keywords you can choose either to add tags/keywords to them all (the original keywords will remain) or to replace all of the keywords or even to remove certain keywords from a batch of images.