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.
Despite the large version number, alien is still (and will probably always be) rather experimental software. It has been used by many people for many years, but there are still many bugs and limitations.
Alien should not be used to replace important system packages, like sysvinit, shared libraries, or other things that are essential for the functioning of your system. Many of these packages are set up differently by Debian and Red Hat, and packages from the different distributions cannot be used interchangably. In general, if you can't uninstall the package without breaking your system, don't try to replace it with an alien version.
Install alien in Ubuntu
sudo apt-get install alien
This will install all the required packages.Now you can start converting your .rpm files to .deb packages.
Available Options for alien
Convert the package.rpm into a package.deb
sudo alien -d package-name.rpm
Convert the package.rpm into a package.deb, and install the generated package.
sudo alien -i package-name.rpm
If you want to keeps alien from changing the version number use the following command
sudo alien -k rpm-package-file.rpm
Suppose we have a avg antivirus avg71lms-r30-a0782.i386.rpm file
To convert .rpm to debian
sudo alien -k avg71lms-r30-a0782.i386.rpm
Now you should be having avg71lms-r30-a0782.i386.deb file
To install .deb file
sudo dpkg -i avg71lms-r30-a0782.i386.deb
If you don't use -k option you should see avg71lms_r30-1_i386.deb file the difference is it will add 1
If you want more available options for alien check alien man page