If anything like how they were before - dead easy. I had one for my old HTC Hero that was just a chunky ass battery with a bigger back to suit. Wouldn't put it past a manufacturer now to make it difficult on purpose though.
You can get 'charging cases' that are loaded with an extra battery, cover the phone, and plug into its charging port as a workaround for sealed devices. There's usually a button that switches the case on and starts charging the internal battery from the external one.
We've opened a piefed instance too - early days yet though: piefed.zip