How to combine concatenate operator on JSON values with AS
Not exactly sure what you are asking, but in Postgres 9.3 or later you can use json_agg() to translate a whole table into a JSON array of records. Prepare the rows in a single subquery and feed whole rows to json_agg() using the table alias:

Multiple layers of nested subselects like you display seem uncalled for.

Simple example:

CREATE TABLE tbl (
  tbl_id serial PRIMARY KEY,
  foo    text
);

INSERT INTO tbl VALUES
  (1, 'First' )
, (2, 'Second');


Query:

SELECT json_agg(sub)
FROM  (
   SELECT tbl_id        AS some_id
        , foo           AS bar
        , tbl_id || foo AS some_derived_column
   FROM  tbl
   ) sub;


Result:

[{"some_id":1,"bar":"First","some_derived_column":"1First"}, 
 {"some_id":2,"bar":"Second","some_derived_column":"2Second"}]


SQL Fiddle.