tpetzke wrote: I'm not aware of a converter that converts chessgui files to something more universal.
Note that WinBoard reads the given format just as easily as any database of PGN games. It triggers on '1.' as indication that a new game starts, in absence of any PGN tags.
So you could simply load it in WinBoard with 'Load Game', specify the desired name of the new book in the Common Engine Options, and select Save Games as Book in the file menu. This would produce a book in Polyglot format.
Note that you would have to start WinBoard in Game Viewer mode to do this, for some reason WinBoard chokes on it if you try to do this with engine.