It is possible to add extra repositories, such as those provided by third parties or those for older version of Ubuntu.
1. Open System → Administration → Software Sources and press Third Party Software.
2. Press Add to add a new repository.
3. Enter the APT line for the extra repository. This should be available from the website of the repository or similar, and should look similar to the following:
deb http://ftp.debian.org etch main
4. Press Add Source and then click Close to save your changes.
5. You will be notified that the information about available software is out-of-date. Press Reload.
6. Most software repositories use a GPG key to digitally sign the files they provide, which makes it easy to check that the files have not been tampered with since their creation. In order for apt to be able to check this, you need the public key that corresponds to the signatures. The key should be available for download on the repository's website.
7. Once you have downloaded the GPG key, import the key by selecting the Authentication tab, clicking on Import Key File, and then selecting the GPG key to be imported.