Does KDENlive support something like lossless rendering? What I mean is, if your video files in your project are all of the some type (including size, samplerate etc.) the output file is on the some type and not changed objects will be added without rerendering.... ?

The video software from Magix calls this option "smart rendering".

The reason why I'm asking for it is, due to my poor hardware I have to convert my MTS files to AVI/Xvid before I use them in KDENlive. Every rerendering doesn't make the videoquality better ;-)

Or does anyone know how to join AVIs using ffmpeg lossless ?

No, Kdenlive does not support this.