It depends on from where you will be importing the articles. You can check the codex to see what existing solutions exist. If you are using custom CMS you can use [X]HTML guide:
<item>
<pubDate>Wed, 30 Jan 2009 12:00:00 +0000</pubDate>
<category>Kites</category>
<category>Taiwan</category>
<title>Fun times</title>
<content:encoded><p>What great times we had...</p><p>And then Bob...</p></content:encoded>
</item>
<item>...