Welcome to the forums.
So you want the array returned by MyGameElementAdditions::CreateTemplates to contain the items as in SuperGameElement::CreateTemplates plus any new items you may want?
In that case you can do this in MyGameElementAdditions::CreateTemplates
static function array<MyTemplate> CreateTemplates()
{
local array<MyTemplate> MyAwesomeList;
//get list with items filled by superclass version
MyAwesomeList = class'SuperGameElement'.static.CreateTemplates();
//add any other items here
MyAwesomeList.AddItem(CreateAwesomeTemplate9999());
}