Yes. You can. I have got Revolution stored on my USB because I've only got a 4GB Slim 360 and got barely any space due to Rezurection, and Call of the Dead and Minecraft.
I would recommend a 16GB USB drive as you will have enough space for all 4 map packs and micro transactions.
In order to use the USB as an Xbox storage device, it MUST be formatted as FAT32 and then insert it into a USB slot on your Xbox, there are 2 slots at the front and 2 at the back.
After you have inserted your USB, go to the dashboard -> Settings -> Storage.
You will see either "USB Drive" or "Portable Device", either way select it and then the Xbox will ask if you want to partition it. (It's only if you want to store music or photos on it)
Then after a few minutes, the 360 will pop up a screen saying if it was successful or not, if it was, anything you download which is too big for your HDD will automaticly download to the USB.
Note: If you remove the USB while playing on a downloaded map, your game may freeze or crash.
If you insert your USB into your PC after doing this, the USB will appear blank, this is to try and stop people sharing their downloaded maps.
If your Black Ops 2 save game is stored on the 360's HDD and the map packs is on the USB, you will have to keep changing storage devices.
Yes you can. I moved all of my DLC maps on my thumbdrive. I can now play on the regular non-DLC playlist when I unplug my thumbdirve. I still have access to my Peacekeeper and I had people asking me what gun I was using and I've seen people pick up my Peacekeeper to try it out.
I suggest you get a 32gb thumbdrive. It should not cost you more than 25 bucks.