You can select all layers in a composition and extend all of them at the same time (Layer > Time > Time Stretch), but some overlapping would occur.
You could also add all of those layers to a new composition and extend that one, this would fix the overlapping.
But I'm guessing what you mean is, extending multiple composition directly on the Project panel. For this, you can use a script from http://www.crgreen.com/aescripts/ called Selected_Comps_Changer.jsx, located almost at the bottom.
You'll be able to modify the settings for all selected compositions at once (duration, framerate, size).