The way you accomplish this isn't exactly the most obvious - but it will work. First select everything on your timeline. Then drag the entire selection over the canvas window and choose overwrite with transition. It will use whatever transition is set as your default transition. If everything goes right you'll have cross dissolves at every edit point. There is the possibility of errors happening depending on how your timeline is laid out... and if you have a lot of tracks of video or improper handles on some of the clips you may run into some problems... but go ahead and give it a shot.
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Paul Escandon
Producer | Director | Editor
Apple Certified Trainer - Final Cut Pro
Oremus Productions
http://www.oremusproductions.com
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Adjunct Professor of Media
John Paul the Great Catholic University