I'm not sure that it is possible to embed WSA's IP in the log. You can find logging options for WSA here. I don't see the option you want here.
What crosses my mind that you could use X-Forwarded-For field for this purpose. WSA can embed additional header in each packet, which can be later analyzed and processed by Firepower, which should provide you exactly the functionality you need. For how to configure XFF on WSA, please see this post. For investigation on Firepower, please see this link.