You need to either:
a) add the property to project.properties inside the plugin (not recommended)
b) put the property in ~/build.properties (affects all projects and plugins)
c) add the property to the command line with -D

What is your reason for doing this? If you are just manually downloading JARs, perhaps it is better to put them in the local repository directly?

project.properties:
maven.jar.commons-logging = ${basedir}/lib/commons-logging.jar

I get this error because the plugin also tries to use it:

Het bouwen kan niet doorgaan, omdat de volgende afhankelijkheden niet achterhaald kunnen worden:

commons-logging-1.0.3.jar; path override doesn't exist: C:\Documents and Settings\Geoffrey\.maven\cache\maven-xdoc-plugin-1.8
\lib\commons-logging.jar

Due to legacy reasons it is not possible to use the repository for now for everything, although we understand it's better to use the repository.

this way it's also flagged in the project.xml as evil