Add a new collation to a Postgres database
You don't need to create a new DB cluster. You can use CREATE COLLATION in Postgres 9.1 or later. Example in the manual:


  To create a collation from the operating system locale fr_FR.utf8
  (assuming the current database encoding is UTF8):

CREATE COLLATION french (LOCALE = 'fr_FR.utf8');



Be sure to read the chapter Managing Collations in the manual to understand the details. In particular:


  Within any particular database, only collations that use that
  database's encoding are of interest. Other entries in pg_collation are  ignored.


I.e., the collation has to match your database encoding to be available.
But there's more. Read it.