The quick answer is you don't.
Packages tend to be developed for specific distribution. They are linked to certain libraries and place files in certain locations. Trying to use a package from another distribution can case problems because of these differences.
Even on very similar families of distribution, slight differences are often enough to cause incompatibilities. Ubuntu packages - for example - are essentially Debian packages. But small differences in the way the two systems are set up are causing them to drift apart. So using an Ubuntu package on a Debian system could mess it up.
A package is just some software arranged into a convenient form for you. So if you want to add something that isn't in your repository you can always make the package yourself from the source.
You don't state what distribution you are using so it's hard to provide you with specific links. But here’s one on creating .debs.
http://blog.mypapit.net/2006/02/crea...b-package.html
And here's one on .rpms
http://www.ibm.com/developerworks/library/l-rpm1/
the second link looks a little old, so you might want to google around for something more relevent. But it should help to give you an idea of what you need to do.
Hope that helps.
