alien is already in EPEL and it makes it quite easy to install it in CentOS 7.
First, enable EPEL following this tutorials.
Then, install alien by
# yum install alien
Then alien should be ready:
# yum info alien Installed Packages Name : alien Arch : noarch Version : 8.95 Release : 3.el7 Size : 223 k Repo : installed From repo : epel Summary : Converter between the rpm, dpkg, stampede slp, and Slackware tgz file formats URL : https://sourceforge.net/projects/alien-pkg-convert/ License : GPLv2+ Description : Alien is a program that converts between the rpm, dpkg, stampede : slp, and Slackware tgz file formats. If you want to use a package : from another distribution than the one you have installed on your : system, you can use alien to convert it to your preferred package : format and install it.