I've done this several times in Joomla 2.5 and later. Simply insert properly structured rows into the k2_items table in your Joomla database. You'll want to make sure each new row that gets added has a properly incremented id, though. Here's an example:
$data =new stdClass();
$data->id = null;
$data->title = $title;
$temp = strtolower($data->title);
$data->alias = str_replace(' ', '-', $temp);
$data->catid = $catid;
$data->published = 1;
$data->introtext = $introtext;
$data->fulltext = $fulltext;
Do the same with all the other K2 fields (there's a bunch of them) until you finally can write the following code:
$db = JFactory::getDBO();
$db->insertObject( '#__k2_items', $data, id );