MacBook Pro using iMac as external display - how to connect iMac keyboard and mouse to MacBook Pro?

At work I have a 27" iMac on my desk. I now use a MacBook Pro so I can be portable. When I bring my MacBook Pro into work I connect it to the iMac using a thunderbolt cable and click CMD+F2 on the iMac keyboard to enable the iMac as an external display for the MacBook. However, once the display is enabled I want to disconnect the keyboard and mouse and connect them to my MacBook. How do I do this? I cannot disconnect the keyboard from the iMac before clicking CMD+F2, the mouse is not so much of an issue.