For openSUSE Factory run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:wrosenauer:devel/openSUSE_Factory/home:wrosenauer:devel.repo
zypper refresh
zypper install alien
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:wrosenauer:devel/15.5/home:wrosenauer:devel.repo
zypper refresh
zypper install alien
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:wrosenauer:devel/15.4/home:wrosenauer:devel.repo
zypper refresh
zypper install alien