AE comps start at time ZERO. You can add time at the end, but not the beginning.
If you have other footage to add at the beginning, you can move the timeline cursor (CTI) to the point where the current footage should begin, select all the layers, then shift-drag until the first layer(s) to appear snap to the CTI.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA
I have the exact same question. Does anyone know of a script that will do this for you?
I already know how to do it manually, but things don't always snap well (example would be if you have a few layers that end within a couple frames of eachother, or a marker, or keyframe that's visible) so you'd have to Really pay attention carefully to make sure you line up the correct end without snapping.