How to update property value of POM.xml in a maven plugin?

[daition]

Hi,
 
I have a question about developing maven plugin.
 
A plugin can get the property value of POM.xml, as "@parameter" annotation does.
But how can I create a new property in maven plugin, which can also be accessed 
in POM.xml? 
That is, how can I update the property value of POM.xml in a maven plugin?
I have searched for the documents, but no example found. Could anyone help to 
give an example?
 
Thanks!
Delos