Say you have a document like below.
And you want to split "hello world" into multiple lines. Doing this using Notepad++ is easy. Bring up Replace dialog (Search | Replace... or CTRL+H), in Find what use hello world\r\n. This matches the entire line including carriage return and linefeed character. In Replace with replace the string with the intended multi-line data, for example, hello\r\n\world\r\n, will replace the line into two. One is hello, followed by world in the next line. And of course, for this to work you need to select Regular expression in the Search Mode.