First, import this module into your ~/.xmonad/xmonad.hs:

 import XMonad.Actions.Submap

Allows you to create a sub-mapping of keys. Example:

    , ((modMask x, xK_a), submap . M.fromList $
        [ ((0, xK_n),     spawn "mpc next")
        , ((0, xK_p),     spawn "mpc prev")
        , ((0, xK_z),     spawn "mpc random")
        , ((0, xK_space), spawn "mpc toggle")
        ])

So, for example, to run 'spawn "mpc next"', you would hit mod-a (to trigger the submapping) and then n to run that action. (0 means "no modifier"). You are, of course, free to use any combination of modifiers in the submapping. However, anyModifier will not work, because that is a special value passed to XGrabKey() and not an actual modifier.

For detailed instructions on editing your key bindings, see XMonad.Doc.Extending#Editing_key_bindings.