[AC Question]

I dont see any icons (while in call) that give me the option...
dont know if it makes any difference but I have StraightTalk as provider.

[hallux]

That might be a question for Straight Talk customer service.

If you look at the picture below (this is the stock Google dialer app which should be the same as your phone uses), you can see the phone icon with the + next to it, this would allow me to add a call to the current conversation.