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
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.