A Ki Focus is a category of implement, like Staff, Holy Symbol and Rod. This would be like asking if a Wizard who multiclassed Cleric could use a Holy Symbol as a Staff. They're just different things.
If you look on PHB3.p62 you'll see that the monk has "Ki focuses, weapons with which you are have proficiency" listed as the implements that they can use. The "Monks and Weapons" section is just an elaboration on this, explaining that you can use weapons as an implement, or use a ki focus. It also elaborates to say that if you use the magical properties of a Ki Focus you still get to apply weapon specific feats if you are using an attack power with the weapon keyword.
You can also use a weapon as an implement, in this case all the appropriate weapon feats apply to implement attacks.
If you use an implement attack with a Ki Focus, there is no way to also benefit from a weapon feat, with a wording like "when you attack with a light blade" like Nimble Blade.
So ultimately, if you multiclass Cleric, you can now use Ki Focus, Holy Symbols and weapons with which you are have proficiency as implements. To compare these 3 options:
- Holy Symbol: can not be used on Weapon attacks, only a problem for melee basic attacks, as no monk attack has this keyword. No weapon feats apply no matter what you hold in your hand.
- Ki Focus: can be used on any attack that has the Implement or Weapon keyword. No weapon feats apply no matter what you hold in your hand.
- Weapon as Implement: can be used on any attack that has the Implement or Weapon keyword. Benefits from any appropriate weapon feat.
Weapons can benefit from Shards as well, so from an optimization standpoint they are wastly superior to other types of implements for most builds.