Silverstripe ListboxField nested dataobjects

Question

I have the following structure that I have to import from an XML feed:

• CompanyType
  • CompanySubType

for example:

• Engineering
  • structural-engineering
  • architectural-engineering

I created both DataObjects, which look like this:

ExpertiseTag.ss

class ExpertiseTag extends DataObject {
    private static $db = [
        'SortID'                        => 'Int',
        'Title'                         => 'Varchar'
    ];

    private static $has_one = [
        'MemberPage'                    => 'MemberPage'
    ];

    private static $has_many = [
        'ExpertiseSubTags'   => 'ExpertiseSubTag'
    ];

    public function getCMSFields() {
        $fields = parent::getCMSFields();

        $fields->removeByName('SortID');
        $fields->removeByName('MemberPageID');

        $fields->addFieldToTab('Root.Main', TextField::create( 'Title', 'Tag naam' ) );

        return $fields;
    }
}

ExpertiseSubTag.ss

class ExpertiseSubTag extends DataObject {
    private static $db = [
        'SortID'                    => 'Int',
        'Title'                     => 'Varchar'
    ];

    private static $has_one = [
        'ExpertiseTag'      => 'ExpertiseTag'
    ];

    public function getCMSFields() {
        $fields = parent::getCMSFields();

        $fields->removeByName('SortID');

        $fields->addFieldToTab('Root.Main', TextField::create( 'Title', 'Tag naam' ) );

        return $fields;
    }
}

Maybe a ListboxField isn't the way to go, but ideally I would like to create a situation where I can see the nested structure and make multiple selections. To continue: On the holder page I created a ListboxField where I want to list both dataobjects intead of one of the dataobjects.

The ListboxField looks something like this:

$fields->addFieldToTab('Root.Expertise', ListboxField::create(
    'ExpertiseTags',
    'Expertise (tags):',
    ExpertiseTag::get()->map('ID', 'Title')->toArray(),
    $this->ExpertiseTags()->column('ID'),
    null,
    true
));

How can I adjust or recreate the code to achieve the situation as described as above? Maybe ListboxField isn't the way to go here.