Script to add tags to multiple items

This is a rough-and-ready version:

tell application "DEVONthink Pro"
		set this_selection to the selection
		if this_selection is {} then error "Please select some contents."
			display dialog "Enter tags to add:" default answer "" buttons {"Cancel", "OK"} default button 2
			set newTag to the text returned of the result
			if newTag is not "" then exit repeat
		end repeat
		repeat with this_item in this_selection
			set current_tags to the tags of this_item
			if current_tags is not "" then
				set new_tags to current_tags & " " & newTag
				set new_tags to newTag
			end if
			set the tags of this_item to new_tags
		end repeat
	on error error_message number error_number
		if the error_number is not -128 then
				display alert "DEVONthink Pro" message error_message as warning
			on error number error_number
				if error_number is -1708 then display dialog error_message buttons {"OK"} default button 1
			end try
		end if
	end try
end tell

Now: how would I need to change this so I can add more than one tag at a time?

You might have a look at the script ~/Library/Application Support/DEVONthink Pro 2/Scripts/Comments/Convert Comments to Tags.scpt.