Open your settings.xml file ~/.m2/settings.xml
(create it if it doesn't exist). Add a section with the properties added. Then make sure the activeProfiles includes the new profile.
<settings>
<!-- ... other settings here ... -->
<profiles>
<profile>
<id>downloadSources</id>
<properties>
<downloadSources>true</downloadSources>
<downloadJavadocs>true</downloadJavadocs>
</properties>
</profile>
</profiles>
<activeProfiles>
<activeProfile>downloadSources</activeProfile>
</activeProfiles>
</settings>