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.