I was asked by a user if Skype For Business (SFB) can connect with external persons using Skype. I know in the past, this interconnection was not possible but I can now positively advise that it is possible.
I am going to demonstrate how you configure the SFB to connect to Skype using email address.
Skype For Business Client
First, open SFB.
Select the tab NEW
Select the Add Contact icon
Select option Add a Contact Not in My Organization

Enter the email of the recipient in the search box:
You will select the recipient from the result list that shows up

Double click the persons identified and select Add this contact.

It will prompt to send a request to the person and show up in the contact list.
Skype Client
Select Contacts –> Select +Contacts

The recipient will do the same from Skype and enter the persons SFB email address.

select Add, to send request and then it will show up in the contact list.
Try sending each other messages and then it will show the status.