I do not know it this is the right place where to post this question.
I have downloaded org.eclipse.wst.xml.ui from Eclipse CVS repository. This plugin is part of the Eclipse XML Editor; I did some changes adding new views, menu and so on.
When I launch plung-in from inside my Eclipse using "Run as Eclipse Application", it works fine and I have all features I have added.
Then I tried to export it generating a JAR file using the Export Wizard.
The file was org.eclipse.wst.xml.ui.vQUALIFIER.jar where qualifier represents the date when jar was generated.
I put it into dropins folder but it is never loaded by Eclipse; Eclipse always loads the default plug-in org.eclipse.wst.xml.ui which is inside the plugins folder.
Version number of my plug-in is higher then the default's one.