Do you mean an example of running an external file from within your game/program made in Construct Classic? Otherwise, I don't believe anyone has made an example of using fre:ac to convert MP3 to Ogg in the forums, but it might have a readme.txt file on how to convert files from a command-line parameter.
This is definitely not an easy task, and would almost be harder to code than to make a simple PDF guide for your users to convert MP3 to OGG themselves. Is it absolutely necessary to convert MP3 to Ogg inside your game/program?