Can an NPC make a declaration of "You have a bad knee" and then make a fists roll to make it true? So that next exchange it can be tagged?
There is an important difference between a maneuver and a declaration. A declaration doesn't change anything, it just puts a spotlight on an aspect that might have already been there but has not been of any interest to the story so far. By declaring it, you make it part of the story. A maneuver on the other hand does indicate a change that is brought on by a character (player or nonplayer alike).
In your case, there are a number of ways this could play out:
If the character really has a bad knee (either as part of his concept or as a consequence) the npc can try an assessment. The difference to a declaration is, that the aspect really does exist, the npc just doesn't know about it, so he can't use it in any way. through an assessment action he can uncover the aspect and use it against the character.
Or he can make a fists/weapon maneuver to put the aspect "Bad Knee" on the character to tag it later in the fight against that character.
Can a wizard cast without benefit of his hands or gestures?
Can a wizard cast without speaking?
First of I would say definitely yes, BUT:
Usually a wizard is not used to casting a spell without a gesture and at least one word. If that is the case, I would place aspects like "hands tied" and "gagged" on him and compel them for -2 shifts on the discipline roll. A wizard that does not use gestures and speech for his evocations would have to have a very good reason not to do so, other then to eliminate the possibility of being compelled on it.