How can I save an iTween animation as a .anim file?

The short answer is: you can't.

The long answer is: The anim files are custom binary files that Unity creates and we don't really even know what goes into them and in what format etc. If you are talking about an iTween path animation, it's e.g. C# code that calculates values based on waypoints and time. Those are 2 very different concepts.

You'd have to reverse engineer/come up with formulas that take in the waypoints you give iTween and convert it to data that causes Unity's animation engine to produce the same motion, then be able to bundle that data into the binary format Unity uses.