The Arch Wiki has information on how to use a different WM - i3wm specifically - in KDE (that’s what I understand from your post, please correct me if I’m wrong).
For this, you’ll have to install the package
manjaro-i3-settings and after its installation, copy the content of the
/etc/skel/ directory to your home directory - make sure to create a backup copy of your home directory first, just to be on the safe side.
After doing so, there might be some issues regarding packages not installed in your system. That’s something you’ll have to figure out on your own.