How can I add a switch into an existing circuit but only control some of the circuit?
As you have drawn it, these lights will be always-on.  Most people do not want that, though it is becoming more popular due to LED lights drawing so amazingly little power.  

Imagine with your mind's eye, if the colors of wires were related to their function.  Start with 


black for the "hot" wire, that is always hot regardless of any switch positions. 
White for the "neutral" wire, which is the normal current return.  


Current flows in loops, so the two wires above are bare minimum.  Now let's think about a third: 


Red for "switched-hot", a wire that is hot when the controlling light switch is on.  


Normally we are stuck with the colors that are in cable, and don't get to color-code like this.  But in this particualr case, I think it will work out.  If not, they make colored electrical tape.  

Now, light switches need all 3 wires.  If you heard that light switches only need black and red, and you can do that with black/white cable, that knowledge is obsolete.  Switches need all 3. 

Lights need 2 wires: switched-hot and neutral (red white). They don't need always-hot (black) unless you want the option to jumper them on 24x7, but it's ok for always-hot to pass through their box untapped.  

So the deal is, you need to run /3 cable (red black white) between all the controlled lamps and the switch.  The switch taps black and red, leaves white capped off for future smart switch use.   The lamps tap red and white, and black passes through.