The latest versions of SCA Maven Plugin aggregate scans by default and so module based maven projects do not need any explicit settings anymore. It is evident from plugin documentation (extract below):
$mvn com.fortify.sca.plugins.maven:sca-maven-plugin:help -Ddetail=true -Dgoal=scan
output:
Available parameters:
aggregate (Default: true)
If set to true, aggregate mode is enabled.
User property: fortify.sca.aggregate
buildId (Default: ${project.artifactId}-${project.version})
Specifies the SCA build ID. The default is project artifact ID and
version. In aggregate mode, the top level project artifact ID and version
is used for all modules.