Here is a simple command to create a submodule from maven-archetype-simple:
mvn archetype:generate -DarchetypeGroupId=org.apache.maven.archetypes -DarchetypeArtifactId=maven-archetype-simple -DgroupId=<your-groupid> -DartifactId=<your-artifact-id>
The parent project's packaging should be <packaging>pom</packaging>