:s/\v(.*)/- \1/g (very magic)
:s/\(.*\)/- \1/g
The above commands do the same thing: adding a dash before the line.
So,
What shall we do with the drunken sailor?
becomes:
- What shall we do with the drunken sailor?
You can open the LibreOffice window completely by pressing the CTRL+Shift+J keys together.