I've kind of been wondering this too. Honestly it can't be that difficult to make whatever they use to do the conversion.
I'd guess...you'd have to cut out that little seperator between the main window and the vent window for starters. Remove the vent window of course...
Then I'd guess that the rail the window sits on is replaced with a longer one to accomodate a bigger window...heck, you could probably get by without a longer rail...anyways,
The last part is the big sized window. Should just see if brothers sells just the window. Call them, tell them you bought the truck from soemone, it already has the one piece window conversion installed but the windows are broken.