If you want tabpages, why not use a TabControl with UserControls inside ? If you need to transform one of these tabs to a floating window, just put the UserControl in a new Window...
Can I answer this question with another question; why would you not create them as controls rather than other WPF windows, that you want to host in the main WPF window?
a bit late on this, but I guess with WindowsForms interop you can put in WPF a WinForms control host and in that host put a WinForms control that hosts the handle of a WPF window