I'm desiging a pygtk GUI and want to embed an external application into it.
Does anyone have any idea how this can be done?
It depends on what application you are trying to embed into yours, but if the other app is a GTK app (or one that supports the XEMBED protocol), you should be able to do this with gtk.Plug and gtk.Socket. The PyGTK tutorial has a section explaining how to do this:
PyGTK 2.0 Tutorial - Plugs and Sockets from Wayback Machine (archive.org)