Great question Gary! There absolutely is a way to do this. When viewing the stats on your campaign, locate the export icon in the top right corner, and make sure to set the export to filtered so it will filter the list you are viewing (in this case viewed or clicked). Then, you can add a column for tags and add the desired tags to the whole list at once.

Once you have the tag(s) added to your spreadsheet, import the spreadsheet, and make sure to replace existing contacts so that it will add the tags to existing contacts. Click Here for a guide to importing your spreadsheet if needed. 

Thanks for your question, leave a comment here if you need more guidance or help with something else