Hi G Douglas,
Welcome to post in our forum.
You could configure federation with external SFB users with Lync server 2013 and you could use all functions of Lync, the following blog describes how to set up Lync federation in Lync server 2013, please refer to
https://technet.microsoft.com/en-us/library/jj204800(v=ocs.15).aspx
Based on my test, after federation, you could add the external users to your Lync contact list (favorites, other contact, etc.)

As a supplement, if you want to configure the federation between Lync server and Skype, so far, between Skype for Business client and Skype software, it is only support peer-to-peer IM, A\V chat, it’s not supported meeting, group chat and sharing PPT\desktop.
The following blog describes how to set up federation between SFB and skype, it’s similar to Lync server 2013, please refer to
https://technet.microsoft.com/en-us/library/dn705313.aspx
Hope this reply is helpful to you.
Regards,
Alice Wang
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