There is a way to do this. Create the dissolve you want in any other sequence, the make this be your defauilt transition. (Drag it to your favourites folder, click on it, then Effects > Set Default. Now gather up your 1800 photos, selecting all of them in your bin. Drag the whole set of them to the Canvas until the pop-up menus appear, and select Overwrite with Transition. Voila.