In a similar context, I have done something like the below. The user can keep the default list, or specify its own list of fields, or add its own fields to the default list.

config :fields, :validate => :array, :default => ["+"]
...
@default_fields = ["field_a", "field_b", "field_c"]
...
if fields[0] == "+"
  fields_ = @default_fields + fields[1..fields.length-1]
else
  fields_ = fields
end