English: Duplicate your Depth and Object Buffer Channel separately to new ps documents, save them, and load them in Affer Effects. Then you'll be good to go.
What the man said. Your workflow is simply crooked. The only format where AE can read custom channels is EXR. If you don't want/ can't re-render from your 3D app, then create an action or script in Photoshop to extract the info.