oltdev

This plugin aggregates multiple blog(s) into one 
single blog. The imported blogs must be in the same WPMU installation 
as the destination blog. Suitable for WordPress MU only.
<h3>How to</h3>
<ol>
<li>
Using your favorite text editor, create a .txt file with all the 
links of the blog you wish to import. Separate each blog in new line. 
You can name it anything you want. Here’s an example.
https://wordpress.org/blog1/ 
https://wordpress.org/blog2/ 
https://wordpress.org/blog3/
</li>
<li>
From the menu you can choose to specify categories and tags filter. 
You can have multiple filter in one line. They are comma seperated.
</li>
<li>
Click on import button and begin the import.
</li>
</ol>

Multiple Import

Aggregate multiple blog(s) into one single blog.

Brad Williams

This plugin lets you create category mappings allowing for one top-level blog to act as a portal for your global community of WPMU blogs. Category postings are aggregated based on the mappings and displayed on the top-level blog.
Features:
<pre><code>* Map multiple top-level categories
* Built-in sidebar widget to display all categories
* WordPress MU 2.7 approved!
</code></pre>
<h3>Plugin Support</h3>
<a href="http://webdevstudios.com/support/wordpress-plugins/" title="WordPress Plugins and Support" rel="nofollow ugc">WordPress MU Category Mapping Plugin</a>

WordPress MU Category Mapping Plugin

Plugin links categories network wide to provide one top-level blog to act as a portal for your global community of blogs

Multiple Import vs WordPress MU Category Mapping Plugin