Realistically, no WPF doesn't really support embedding windows inside other windows. I believe Microsoft decided that having this sort of hierarchy (MDI) was bad practise, so generally the idea is that you create user controls for the content.
For what purpose do you need the WindowInteropHelper
]? If it's to retrieve the handle of the window once embedded in your parent window, then that's likely not going to be possible so you probably don't need it.
If you're after resolving handles for elements in a window, you need to know that WPF doesn't work like a standard WinForms application. All elements in a WPF window are part of a visual hierarchy, so identifying the handle for a user control or similar will simply return the handle of the parent window.
As far as your AvalonDock solution is concerned, I believe they have their own mechanism to produce new windows if a floating window is required. In the event that the control does this, you could probably then use the
to target the new floating window.