Trigger function to change data to correct datatype

Hi list !

I have a MSAccess table in which dates are stored as strings. Some
dates are null but are stored as '-', and I cannot change this
because many queries use this value.
I need to insert theses values in a PostgreSQL table, but with a
real 'date' datatype.
Since '-' is not correct, I thought about creating a trigger that
would change the '-' to NULL before the INSERT took place, but my
function does not work :

CREATE OR REPLACE FUNCTION check_date()
   RETURNS "trigger" AS
$BODY$
   BEGIN
     IF NEW.mydate = '-' THEN
       NEW.mydate = NULL;
     END IF;
   END;
$BODY$
LANGUAGE 'plpgsql' VOLATILE;

And :

CREATE TRIGGER check_mydate
   BEFORE INSERT
   ON mytable
   FOR EACH ROW
   EXECUTE PROCEDURE check_date();


But when I try to insert a row in this table I have an error :
ERROR:  invalid input syntax for type date: "-"

I would like to avoid using a function in the INSERT to replace
the "-" by NULL, because I execute this query on linked tables in
MSAccess, and since both table have the exact same structure, I
use the syntax 'INSERT INTO psql_table SELECT * FROM msaccess_table'.

Is there a way to achieve this ?

Thanks for helping !

Regards
--
Arnaud