You can CTRL+Click the mask in the layers palette. That will create a selection that you can feather. Afterwards, you simply need to fill the selection.
However, I'm having trouble explaining how to affect your mask properly.
A quick and dirty way would be to duplicate your layer+mask. Delete the mask from the original, CTRL+Click the copy's mask, Feather the selection, return to your original layer, create a mask from the selection and delete the copy (since you don't need it anymore).
Once again, there are certainly cleaner ways to do this but this is what comes to mind right now.