I have similar issue.

I have News Holder and News Post. But I Have extended Blog Holder and Blog Entry respectively.
Can I change this to DataObject while inheriting from Blog Entry?

I am a designer who can manage to code with little help :)

Current Code for NewsPost is

class NewsPost extends BlogEntry {
	static $db = array (
     'GalleryImageWidth' => 'Int',
     'Featured' => 'Boolean',
     'UserPost' => 'Boolean',
   );
	
	static $default_parent = 'NewsHolder';
	static $can_be_root = false;
	
	static $has_one = array(
		'Photo' => 'Image'
      
		//'Gallery' => 'ImageGalleryPage' 
		//'GalleryImages' => 'GalleryImage' 
	);
	
	static $has_many = array (
          'GalleryImages' => 'GalleryImage'
    ); 



  public function init() {
    parent::init();

    // Note: you should use SS template require tags inside your templates 
    // instead of putting Requirements calls here.  However these are 
    // included so that our older themes still work   

    Requirements::block("sapphire/thirdparty/behaviour/behaviour.js"); 
	Requirements::block("sapphire/thirdparty/jquery/jquery.js");
	
  }

	function getCMSFields() {
		
	$fields = parent::getCMSFields();
    $fields->addFieldsToTab("Root.Content.Main", new CheckboxField('UserPost', "Post by a Member"));
	$fields->addFieldToTab("Root.Content.Main", new CheckboxField('Featured', "Featured Post"));
	$fields->addFieldToTab("Root.Content.Images", $leadImage= new ImageField('Photo', "Upload your lead image here"));
		$leadImage->setFolderName('Uploads/' .date("Y").'/' .date("m"));
	
	$fields->addFieldToTab("Root.Content.Configuration", $myField= new NumericField('GalleryImageWidth', 'Image Width'));
		/*$fields-> addFieldToTab ("Root.Content.Gallery", new DropdownField("GalleryID", "Gallery", 
			DataObject::get("ImageGalleryPage")->toDropdownMap(),  'Content'));*/
		
		$manager = new ImageDataObjectManager(
         $this, // Controller
        'GalleryImages', // Source name
        'GalleryImage', // Source class
        'MyGalleryImage', // File name on DataObject
         array(
            'GalleryImageTitle' => 'GalleryImageTitle'
         ), // Headings
         'getCMSFields_forPopup' // Detail fields (function name or FieldSet object)
         // Filter clause
         // Sort clause
         // Join clause
      );
      $fields->addFieldToTab('Root.Content.GalleryImage', $manager); 
     return $fields;
    }

	
// Function for SideBar
    function GetSidebarContent($num=3) {

      //$randomNumber = 
      $sidebarContent = DataObject::get_one("SidebarContent");

      if(!$sidebarContent)
      {
        return false;
      }

      $contents = array();

      $contents[] =  DBField::create('Varchar', $sidebarContent->Content1);
      $contents[] =  DBField::create('Varchar', $sidebarContent->Content2);
      $contents[] =  DBField::create('Varchar', $sidebarContent->Content3);
      $contents[] =  DBField::create('Varchar', $sidebarContent->Content4);
      $contents[] =  DBField::create('Varchar', $sidebarContent->Content5);

      shuffle($contents);
            
      //return new DataObjectSet(array_rand($contents, $num));      
      return new DataObjectSet(array_slice($contents, 0, $num));   
      //return new DataObjectSet($contents);   
  }

	
}
 
class NewsPost_Controller extends BlogEntry_Controller{


}


/**
 * Replaces Images to a predefined max width and adds a nyroModal class to the image
 * In template use : $Content.Parse(ArticleImageParser)
 */
 
class ArticleImageParser extends TextParser{
	
	public function parse() { 
		
		// match image src
		$pattern = '/<img[^>]+src[\\s=\'"]';
		$pattern .= '+([^"\'>\\s]+)/is';
		if(preg_match_all($pattern,$this->content,$match)){
			$i = 0;
			foreach ($match[0] as $m){	
				//var_dump($m); //"<img class="right" src="assets/Articles/installingsilverstripe/wampserver.jpg"
				
				// get original image url
				$href = preg_replace('/_resampled\/resizedimage[0-9]*-/', '',  $match[1][$i]);
				// add a tag with original image url
				$this->content	= str_replace ($m,'<a class="popImage" rel="grp-all" href="'.$href.'">'.$match[0][$i],$this->content);
				$i++;
			}
			
			// add missing </a> tag after each image
			$this->content = preg_replace('/<img[^>]+>/', '${0}</a>', $this->content);
		}
		
		return $this->content;
	}
}

?>