Open BrowserSim simulated device. Right-click device and click Preferences. For devices, click Add. In last field of Add Device wizard user selects a skin from a predefined list.

User should be able to select to upload a custom skin. For example they might have their own skin for a pink iPhone 5c or some other shiny new phone.

I guess the issue in including this feature might be activating functionality for buttons on the front of the skin? Perhaps that's why it hasn't been done?