In Notepad++, use Search->Replace
Find: \r\n
Replace: \r\n\r\n
Check the Extended item in the Search Mode at the lower left.
The above finds all of the carriage return/line feed combinations (\r\n) and doubles them, causing a blank line to be added between paragraphs.
The Notepad++ View->Show Symbol->Show All Characters menu will allow you to see the characters before and after the replacement is done, as is displayed in the image below (after replacement was completed).