Replace characters while fetching news to MOBI format

Is there a way to search/replace characters while fetching news to MOBI format?

I download various news sources with Calibre to MOBI format, which I then save to my Kindle Paperwhite. I find MOBI's table of contents navigation far superior to EPUB.

Specifically, I want to replace "—" with " -- " because words to the left or right of an em dash cannot be selected for Kindle's dictionary lookup.