From https://help.ubuntu.com/community/RPM/AlienHowto , use the alien package:
Alien converts an RPM package file into a Debian package file or Alien can install an RPM file directly. This is not the recommended way to install software packages in Ubuntu. If at all possible, install packages from Ubuntu’s repositories using Add/Remove, apt-get, or the Synaptic Package Manager. Package dependency conflicts may occur when attempting to install RPM packages.
These commands should do it:
sudo apt-get install alien
sudo alien -i package_file.rpm
And from alien’s man page :
WARNING
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.