I have a Sanyo FH1 camcorder and I convert the recorded files to DNxHD. It's better/faster to edit. But converting means quality loss so I thought: why not rendering with the original file? I don't know if it is possible but I'm thinking about it and maybe it's an option. Or is this option already implemented?

similar things have been suggested (with other codecs). try searching for proxy editing, but AFAIK it hasn't been done yet.

Will it work when I edit a saved project with a text-editor and rename all converted DNxHD files from *.mp4.mov to the original *.mp4 ?
I tried it and it seems to work but maybe I'm wrong.

It should work, but conversion to DNxHD is not much quality loss for most people to bother.