Alien should not be used to replace important system packages, like init, libc, or other things that are essential for the functioning of your system. Many of these packages are set up differently by the different distributions, and packages from the different distributions cannot be used interchangeably. In general, if you can't remove a package without breaking your system, don't try to replace it with an alien version.
Ubuntu user can install alien using following command: apt-get install alien
OpenSuSe user can install alien using "1-click" installer - here
Fedora user can install alien using command: yum install alien
Convert the package.rpm into a package.deb
alien --to-deb package.rpm
Convert the package.deb into a package.rpm
alien --to-rpm package.deb
Convert the package.rpm into a package.deb (converting to a .deb package is default, so you need not specify --to-deb), and install the generated package.
alien -i package.rpm
alien --to-deb --to-rpm --to-tgz --to-slp foo.deb bar.rpm baz.tgz
Creates 9 new packages. When it is done, foo bar and baz are available in all 4 package formats.