Extend duration of imported static layers in After Effects
Open the imported comps. Not in the layer source window, just as comps (i.e. double click from the project panel).

Go to the composition settings menu and change the duration to whatever you want.

Drag the out-points of all the layers to the end of the comp (hint, press the end key on your keyboard and then with all the layers selected hit alt+]).

In your main composition you will now be able to drag the end of the photoshop comps to their new length.