help with sintaxis of quotes on function

Dear Sirs:

Please I am attempting implement a function that update a column to current time + X seconds

So the query involved looks like:

update mytable set mytime=now+'X seconds';

Here X is a integer value,

Now I need implement this on my function, But I have problems at implement this at quotes, e.g. my function could looks like:

CREATE FUNCTION addtime(integer) RETURNS INTEGER AS '
DECLARE
additional ALIAS FOR $1;
BEGIN
update mytable set mytime=now()+''additional seconds'';
END;
'
return 1;
LANGUAGE 'plpgsql';

I know probably this doesn't works, justly I have doubts about how to implement this since I want the funtion takes the variable additional , not the string 'additional', What I should to consider? I read the documentation on pqsql language aboute using quotes but none case is resemblance to my case.

Can someone tell me an orientation about this?

Thank you in advance.

Ernesto