As Nicole brought up, hugels shrink and settle so you do have to make additions for at least the first few (read as 3-5) years.
Depending upon how much
wood you used to build the hugel, you might be making these additions for up to ten years.
As both Nicole and Michelle brought up, compost and soil are the items to use for this build up and then you mulch.
Once this is all done you can plant right through the mulch layer.
Redhawk