GTK Popup-Window in Motif/Xt Application

[Karsten Chmielewski <my_last_name rrzn uni-hannover de>]

    Dear GTK-developer community,

I am maintaining and enhancing an application running as a plugin for internet
browsers.  The embedding into the browser is done via the standard way, i.e.,
via the XtWindowToWidget call.  The application creates popup-windows.  So far
Motif and the X-Toolkit has been used for these windows.  However, I would
like to switch to GTK, but miss the following information so far:

  * It is not practical to port the whole application to GTK at once.  I would
    like to gradually switch to GTK by only adapting those popups which need
    revision from the application point of view.

  * How do I keep the main embedded window as it is and just create
    popup-windows using GTK?

Formulated in another way this is: How do I create GTK-style popup-windows
from a Motif/Xt based main application?

Thanks for any information,
Karsten Chmielewski