Hmmm there seems some tricky bits I found this
http://www.giantitp.com/forums/showsinglepost.php?p=14931255&postcount=2
(1) Ki focuses in general: Ki focuses are implements, first and foremost. If you are using an Implement power, you can use a Ki Focus for it so long as you're proficient in them.
They also "share" their enchantments with any weapon you're using. So if you have just a normal longsword and a +3 Ki Focus, you get to use the Ki Focus's +3 on your attacks with it. This is awesome for characters like Executioners who expect to swap weapons a lot. It's likewise awesome for unarmed guys and guys who use improvised weapons, like Arena Fighters and some Brawler Fighters, because it affects anything they pick up. (Interestingly, Ki Focus Expertise gives its feat bonuses to any Implement attacks made with a Ki Focus and any Weapon attacks made which are enhanced by the Ki Focus.
(2) Monks: Okay, so monks are proficient in Ki Focuses. They can do all that stuff I just mentioned. Critically, however, Monks can also use any weapon they are proficient in as an Implement. So they don't even need a Ki Focus, if they don't want one. If they're proficient in a Spear and pick up a Magic Spear +2, the Spear's properties and bonuses work towards the Monk's implement attacks. It's really pretty sweet.
Finally, there's a case where you have both a magic weapon and a magic ki focus. Simply put, they don't stack. If I have a Ghost Touch Ki Focus and an Alfsair Spear, it's one or the other when I make a weapon attack with the spear. (And if I have Ki Focus Expertise but not Spear Expertise, it's best if I use the former to get my Expertise bonuses.)