Respostas:
Tente o seguinte:
ALTER TABLE table ADD COLUMN col1 int, ADD COLUMN col2 int;
ALTER TABLE table ADD COLUMN col1 int default 0, ADD COLUMN col2 text default 'foo';
ALTER TABLE IF EXISTS TABLEname
add ADD COLUMN IF NOT EXISTS column_name data_type [column_constraint];
consulta detalhada em que column_constraints são opcionais
ALTER TABLE [ ONLY ] name [ * ] action [, ... ]
, postgresql.org/docs/current/static/sql-altertable.html