I've configured plasma desktop, it seems that all settings are kept in
~/.kde4/share/config/plasma-appletsrc . If I put that file to newly
created user, changes are applied on its desktop.

I'd like to make this settings system-wide, so I put plasma-appletsrc
file to $( kd4-config --prefix )/share/config/ , and deleted ~/.kde4
from test user $HOME . I've got no effect, no changes are applied it
seems that global plasma-appletsrc is ignored.

It it possible to make desktop settings global so they will be applied
to all users (unless they override them on ~/.kde4)?

You may also need to include the matching plasmarc and plasma-desktoprc files. I am not sure if that is the correct location however.