Verificando a ajuda:
$ add-apt-repository --help Usage: add-apt-repository <sourceline> ... Options: -h, --help show this help message and exit -m, --massive-debug Print a lot of debug information to the command line -r, --remove remove repository from sources.list.d directory -s, --enable-source Allow downloading of the source packages from the repository -y, --yes Assume yes to all queries -n, --no-update Do not update package cache after adding -u, --update Update package cache after adding (legacy option) -k KEYSERVER, --keyserver=KEYSERVER Legacy option, unused.
Então:
sudo add-apt-repository -n ppa:...