If you have in your system the possibility to log-in with different sessions, at the left side of the "Sign in" button, there is a button (with a cog icon) that allows you to select the session. But that button is not included on the navigation focus chain. 

FWIW: if you open it with the mouse, and put the selection on one of the items, you can use the keyboard to move up and down and change the selection. So that menu is keyboard navigable. As said, the only not acccessible via keyboard is the button itself.