You can add a vector mask, not a second raster mask, but that won't get you where you need to go.
There a several ways to add your grunge effect, but probably the simplest would convert the fence layer to a Smart Object (they were new in CS3, from what I remember) and mask that. You'll not interfere with the mask in the object itself, but you will have a problem if you ever need to move either the man or the fence.
Another way to grunge up only the fence layer would be to make a new layer group with only that one layer in it, and add a mask to the layer. If you "unhook" your existing mask from the layer, you will be able to adjust the position of the layer without affecting the illusion.
All that said, a different but possibly much faster technique is:
Make the "man" layer active, then Ctrl-click (Cmd-click) the fence layer's mask icon in the layers panel to load it as as selection.
Copy (Ctrl/Cmd-J or Layer > New > New Layer Via Copy) the man and the background bits seen through the fence onto a new layer.
Move this new layer above the fence layer, and you can do anything you want with that mask.