I'd like to add a faster processor to my MacBook Air 2014 or my MacBook 5,1 unibody. Is this possible at all?
Nope. MacBook Air processors (and RAM and storage, at least in that model) are soldered to the logic board, so there is no way to remove that processor from the logic board without destroying both of them.
Short answer: no, impossible. Long answer: yes, but it's almost impossible. Processors are soldered to the mainboard in almost every laptop. It is possible to unsolder it and re-solder a powerful one (like the 2.66ghz from a 15" 2008 Macbook pro). This process is called "reballing". But it will be expensive, and not so reliable. It's better to do it only if you are having problems with your processor and you have to reball anyway. It's better if you forget about it, believe me.