I know this is far in the future but I still had a issue with this and the fix is simple enough hope it helps.
You merely have to create a button element and place it between the sidebar and pusher elements then assign the correct css class
<div class="ui sidebar"></div>
<button class="ui black big right attached fixed button">
My Button attached to sidebar</button>
<div class="pusher"></div>
Then the button will attach and flow with a sidebar that is on the left of the page.