Who I have the same questioned this topic

My daughter has the Just Kids plan, I have Smart Family app to control her contacts & usage.  She has many friends that do not have cell phone plans yet & only have email addressses to use their phones with over wifi.  The Just Kids plan allows 20 contacts & I put them in with the Smart Family app.  The app only allows telephone numbers & will not allow me to put in email addresses of her friends that do not have phone numbers.  How can I get those email addresses put in on the APP for trusted contacts? I do not want to go through Settings>Messages>iMessage, since that is already active on her phone.