<p>If you use the command <code>add-apt-repository</code>, all your custom ppa are added to the folder <code>/etc/apt/sources.list.d/</code>.</p>
<p>It contains all information for this ppa. So just back up this folder, restore it and you're done:</p>
<ol>
<li><code>tar -cvjf mysweetppas.tar.bz2 /etc/apt/sources.list.d/</code></li>
<li><code>sudo tar -xvjf mysweetppas</code></li>
<li><code>sudo apt-get update</code></li>
</ol>
<p>To re-use the ppa you defined in <code>/etc/apt/sources.list</code>, you can do the following as root (!):</p>
<ol>
<li><code>cp /etc/apt/sources.list /media/[your external drive]/</code></li>
<li><code>cat /media/[your external drive]/sources.list >> /etc/apt/sources.list</code></li>
<li><code>apt-get update</code></li>
</ol>
<p>Note that you will have duplicates in your sources list! So review it before installing new software!</p>
<p>Good luck!</p>
