[M2] System properties

Hello.  Does anyone know if there is a way to declare system properties from within a pom file, rather than doing it on the command line? (Ex: mvn -Dproperty=value scm:checkout)  I saw some examples on the web, some plugins do the following under <configuration> of <plugin>:

<systemProperties>
   <property>
      <name>property</name>
      <value>value</value>
   </property>
</systemProperties>

I am writing a scm-provider, and I want to use the System.getProperty() method in the provider to get the properties.  It works when I declare the properties on the command line, but not when I declare them from inside the pom...  

Any idea?  Thanks a lot!