I add the <dc:format>ePub</dc:format> tag using a clip all the time. I had to add blank spaces (CR/LF) before and after for it to insert as a new line. eg.
Code:
{CR/LF}<dc:format>ePub</dc:format>{CR/LF}
Unfortunately, you can't see the CR/LF.... but if you highlight from the end of one line to the beginning of the next (in the opf) you will get the CR/LF that Sigil uses. Then copy/paste that in the clip editor.