From faustus@CS.Berkeley.EDU (Wayne A. Christopher):
Create the Tk toplevel window but don`t map it (`wm withdraw
.toplevel`). Then re-parent the window to be a subwindow of your other
one and then map it. I have done this when the Tk application is a
separate process, but if it`s the same process I think you will get into
trouble with the event loop, since each toolkit wants control.
*Note:* Tk8 provides better access to this functionality, a spin-off
from making the Tk plugin work.