How to add "on delete cascade" constraints

Hello,

in 8.4.9 is it please possible to add "on delete cascades" to
the both foreign keys in the following table w/o dropping the table?

# \d pref_scores
         Table "public.pref_scores"
 Column  |         Type          | Modifiers
---------+-----------------------+-----------
 id      | character varying(32) |
 gid     | integer               |
 money   | integer               | not null
 quit    | boolean               |
 last_ip | inet                  |
Foreign-key constraints:
    "pref_scores_gid_fkey" FOREIGN KEY (gid) REFERENCES pref_games(gid)
    "pref_scores_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id)

There referenced tables are below:

# \d pref_games
                                     Table "public.pref_games"
  Column  |            Type             |                        Modifiers
----------+-----------------------------+----------------------------------------------------------
 gid      | integer                     | not null default
nextval('pref_games_gid_seq'::regclass)
 rounds   | integer                     | not null
 finished | timestamp without time zone | default now()
Indexes:
    "pref_games_pkey" PRIMARY KEY, btree (gid)
Referenced by:
    TABLE "pref_scores" CONSTRAINT "pref_scores_gid_fkey" FOREIGN KEY
(gid) REFERENCES pref_games(gid)

 \d pref_users
                Table "public.pref_users"
   Column   |            Type             |   Modifiers
------------+-----------------------------+---------------
 id         | character varying(32)       | not null
 first_name | character varying(64)       |
 last_name  | character varying(64)       |
 female     | boolean                     |
 avatar     | character varying(128)      |
 city       | character varying(64)       |
 login      | timestamp without time zone | default now()
 last_ip    | inet                        |
 logout     | timestamp without time zone |
 vip        | timestamp without time zone |
 mail       | character varying(254)      |
Indexes:
    "pref_users_pkey" PRIMARY KEY, btree (id)
Referenced by:
    TABLE "pref_cards" CONSTRAINT "pref_cards_id_fkey" FOREIGN KEY
(id) REFERENCES pref_users(id)
    TABLE "pref_catch" CONSTRAINT "pref_catch_id_fkey" FOREIGN KEY
(id) REFERENCES pref_users(id)
    TABLE "pref_chat" CONSTRAINT "pref_chat_id_fkey" FOREIGN KEY (id)
REFERENCES pref_users(id)
    TABLE "pref_game" CONSTRAINT "pref_game_id_fkey" FOREIGN KEY (id)
REFERENCES pref_users(id)
    TABLE "pref_hand" CONSTRAINT "pref_hand_id_fkey" FOREIGN KEY (id)
REFERENCES pref_users(id)
    TABLE "pref_luck" CONSTRAINT "pref_luck_id_fkey" FOREIGN KEY (id)
REFERENCES pref_users(id)
    TABLE "pref_match" CONSTRAINT "pref_match_id_fkey" FOREIGN KEY
(id) REFERENCES pref_users(id)
    TABLE "pref_misere" CONSTRAINT "pref_misere_id_fkey" FOREIGN KEY
(id) REFERENCES pref_users(id)
    TABLE "pref_money" CONSTRAINT "pref_money_id_fkey" FOREIGN KEY
(id) REFERENCES pref_users(id)
    TABLE "pref_pass" CONSTRAINT "pref_pass_id_fkey" FOREIGN KEY (id)
REFERENCES pref_users(id)
    TABLE "pref_payment" CONSTRAINT "pref_payment_id_fkey" FOREIGN KEY
(id) REFERENCES pref_users(id)
    TABLE "pref_rep" CONSTRAINT "pref_rep_author_fkey" FOREIGN KEY
(author) REFERENCES pref_users(id)
    TABLE "pref_rep" CONSTRAINT "pref_rep_id_fkey" FOREIGN KEY (id)
REFERENCES pref_users(id)
    TABLE "pref_scores" CONSTRAINT "pref_scores_id_fkey" FOREIGN KEY
(id) REFERENCES pref_users(id)
    TABLE "pref_status" CONSTRAINT "pref_status_id_fkey" FOREIGN KEY
(id) REFERENCES pref_users(id)

Regards
Alex