Ubuntu is Debian based and therefore uses
.deb packages to install. If you want to install
.rpm packages, you first should convert them into
.deb packages with a conversion software such as
alien. Then you can use
dpkg to install them.
However, caution must be applied by doing that. Even such conversion software does not always create the correct dependencies to other packages. Therefore, the software installer might refuse to install your package because of missing dependencies, or the software might not work very well because not all dependencies have been created and maybe some libraries are missing.
Usually, it is better to find a
.deb package if at all possible.