Hi,

From Template settings you cannot. But you have 3 alternatives:

1. You can edit Helix Ultimate plugin code (php) and add what you need. I talk about file: plugins\system\helixultinate\overrides\layouts\joomla\content\social_share.php
2. OR, you can disable default Social Share icons from Template Options, then find on JED Social Share plugin (that have all you need), install and publish it.
3. Or, you can hire somebody who will write you a simple social share plugin that include needed icons.

2 and 3 is better than (1) becuase you don't have to be worry about Helix updates that will delete your work/modications.

For today, it's all I can help.

Thanks Paul