add custom parameters to map.resources


lets say I defined the following resources:

map.resources :profiles

Is it possible to add a custom parameter just to one action via a
block in the map.resources statement? I found the following solutions:

map.resources :profiles, :path_prefix => '/

Problem here: It applies to every action, but I want it e.q. only for
the 'new' action.

map.profile '/profile/:username', :controller => 'profiles', :action
=> 'show'

This works for one action, problem here: There still exists the route
from map.resources to 'show', which would be something like '/

Any suggestions?