trigger to insert on update to non-existing row?

Hello,
I want to create a trigger/procedure to insert the values if I try to
update a row that doesn't exist...

example:
hours_table:
login varchar(16),
hour_type int4,
hours float4

So if I try:
UPDATE hours_table SET hours = hours + 8 WHERE login = 'johndoe' AND
hour_type = 10;
I want it to check:
if a row exists "WHERE login = 'johndoe' AND hour_type = 10" {
do the update.
} else {
INSERT INTO hours_table VALUES('johndoe',10,0);
& do the update
}

I've created a procedure that will take input of login & hour_type and
return hours:

CREATE FUNCTION get_hours(text,int4) RETURNS float4 AS
'SELECT hours FROM hours_table WHERE login = $1 AND hour_type = $2;'
LANGUAGE 'sql';

That part works.

I think the trigger should look like this:

CREATE TRIGGER upd_hrs BEFORE ON UPDATE ON hours_table
FOR EACH ROW EXECUTE PROCEDURE need_insert();

What should need_insert() look like? How do I get the values for login and
hour_type out of the update ?

CREATE FUNCTION need_insert() RETURNS bool AS
'
declare
hours float4;
begin
hours = get_hours($1,$2);
if hours ISNULL then
INSERT INTO hours_table VALUES($1,$2,0);
return false;
end if;
return true;
end;
'
LANGUAGE 'plpgsql';

Thanks,
Doug.