In Writer 7.2.5.2 by default there are settings in AutoCorrect that do the following:
:–: (in words, ColonHyphenHyphenColon produces an en-dash.)
:—: (in words, ColonHyphenHyphenHyphenColon produces an em-dash.)
For the em-dash this is pretty good as you can have the em-dash with no space before and after the em-dash. Or you can easily place the em-dash with a space before and after the em-dash. (Different publishers seem to have different conventions on how they like placement of em-dashes.)
In any case, I found this way of doing it so useful that I wrote an AutoHotkey script that gives me the same functionality in any word-processor that I use on my system. In this way I only have to remember one way of doing things for all word-processors on my system.