Nupedia was a low activity open content, international, peer reviewed online encyclopedia project formerly run by Larry Sanger. Sanger also started Wikipedia alongside Jimmy Wales when he got the idea of supplementing Nupedia with a less formal "wiki" online encyclopedia project. So there's a historical connection between the projects. Both Nupedia and Wikipedia are originated by Bomis, a web portal company, so a wider management connection exists.
There's no editorial connection between the projects, however. The participants in Nupedia don't necessarily condone the contents or methods of Wikipedia - and vice-versa.
Note: that Nupedia has been offline since 2003. There are no plans to resurrect it.
To add a Nupedia article to Wikipedia, all you have to do is find the article, copy the contents, and paste it into the appropriate location (whatever you think is appropriate). Nupedia content is released under the GNU Free Documentation Licence (GFDL). You should note that Nupedia is the source of the article and provide a link to the original. For example:
You may not add articles from the Nupedia chalkboard to Wikipedia, unless you have permission from the original author. Content on the Nupedia chalkboard is not covered by the GFDL.
However, content in the Nupedia articles-in-progress (see http://www.nupedia.com/aip.phtml ) do appear to qualify under the GFDL (see talk and [1]) so they can be ported to Wikipedia. An example citation might be:
See: http://www.nupedia.com/aip.phtml Web Archive version 2003-08-10