In options/editing under default overlap add a time you want
your transition to last then add all photos at once and they will all be crossfaded. Then select all and drag a transition of you choice
to the transition area. All the selected pics will have the same
transition.
And from the help menu!!

Add a transition to all selected events
Life has enough tedium. If you tend to use the same transitions often, you can save yourself some time by adding a transition to all selected events in one fell swoop.

Select the events where you want to add the transition.

From the View menu, choose Transitions to display the Transitions window.

Select a transition from the list on the left side of the window. The thumbnail images on the right side of the window represent each of the existing presets for the selected transition. Hover your cursor over a preset to see an animated example.

After you've found the setting that you want to use, drag it to the a transition on one of the selected events. The new transition will replace all existing transitions in the selected events.

The Video Event FX dialog is displayed to allow you to edit the transition's settings, and a is displayed in the timeline to show you where the transition takes place. You can also click this icon to edit the transition's settings.