Take a look at the JSON built-in functions that are implemented in Postgres (http://www.postgresql.org/docs/9.3/static/functions-json.html) and implement the same for Phoenix in Java following this guide: http://phoenix-hbase.blogspot.com/2013/04/how-to-add-your-own-built-in-function.html
Examples of functions include ARRAY_TO_JSON, ROW_TO_JSON, TO_JSON, etc. The implementation of these built-in functions will be impacted by how JSON is stored in Phoenix. See PHOENIX-628. An initial implementation could work off of a simple text-based JSON representation and then when a native JSON type is implemented, they could be reworked to be more efficient.