You will have to use OAuth or similar plug in from marketplace. Una.io should support it (if you are directing your website users to una.io [probably you might be talking about your own una.io based website, so it would be nice to know what's it]) 

If you are talking about yourwebsite dot com to una dot io login, then its not that easy [But can be done using OAuth or similar plugin]. 

If you control both website codes / databases, then there is way to do it with little customization. 

Anyways, Need more details about your use case to answer it correctly.