Add a 2D texture to a 3D texture

How would I go about copying a 2D texture into a certain depth of a 3D texture?

Would I use glCopyTexImage2D?

You can use glTexSubImage3D or glCompressedTexSubImage3D. I guess your x and y offsets would be 0, and the z offset would be the layer.

On NVIDIA hardware you could use the NV_copy_image extension.