You cannot just mix
and WPF code; those libraries have very different nature. More exactly, you can use Forms controls in WPF application or WPF controls in Forms application, but this is done via the special interop layer which is very complex in its architecture, but it can resolve such problems, which are really quite difficult.
You can start with this Microsoft's walkthrough article: http://msdn.microsoft.com/en-us/library/ms751761.aspx
This CodeProject article can also be useful: Winforms / WPF Interop
However, I would rather advise you to think: do you really need all involved complexity and the hassles? First of all, thing about using pure WPF; usually, this is quite possible.