Disputed.

I've seen rogue-wizard MC builds that use the dagger as an implement and because the rogue is "using" the dagger in the attack you may still apply sneak attack. This would the exact same thing with the ki focus. The ki focus is channeling its energy to the dagger which is being used as the implement.

However, [there is significant discussion][1] as to whether this works in the context of the assassin. 

I would say that it works, however depending on how the ki focus rules are read it could definitely be ruled the other way.


  [1]: http://community.wizards.com/content/forum-topic/2804461