While development of any big portal, we need to customize control panel also. In such a case, its generally needed to make a new category in control panel and adding portlet in it. So, here are the steps, hope you'll find it helpful.
1. Override the class PortletCategoryKeys (in ext-impl\src\com\liferay\portal\util\PortletCategoryKeys.java)
Put the category in ALL array in the order you wish to appear your category.
2. Now add portlet in this category using the following entry in liferay-portlet-ext.xml file
here portlet_id is the id of the portlet that you wish to add in our custom category. Specify the name of the category in <control-panel-entry-category> entry and sequence of portlet appearance in determined through <control-panel-entry-weight>. The higher the weight, upper will be the portlet(weight can also be in floating number).
3. Now an important point, we must enter the category name entry in language-ext.properties file, so that our category can be recognized, otherwise you'll get category name as "category.mycategory"
That's all I did, hope will work for you too.