The only one I can find that was actually converted and used as a freighter was the old German coast defense ship SMS Frithjof.
http://en.wikipedia.org/wiki/SMS_Frithjof She was pretty much gutted from the sounds of it, with U-boat engines replacing her existing set. No idea what the conversion looked like though, can't find any pictures.
Yeas found that aswell! Too bad there are no any info about it. Don't forget the French cruiser ( sold to Peru in 1912? whish were resold to Belgium and become the freigther Peruvier!