Is it possible to add encryption capabilities to an already existing table?

I'm using pgcrypto module for PostgresSQL, and in the document it was indicated that this line:

CREATE EXTENSION pgcrypto;

should be executed before creating the table which stores encrypted data. My question is, if the table has already been created without the above mentioned line being executed, can I modify the table later by running this line to give it the encryption ability? My goal is to use *pgp_sym_encrypt()* for INSERT operations, *pgp_sym_decrypt()* for SELECT operations and both functions for UPDATE operations. For now I can get prepare statement to work, but exec would fail, I guess because the CREATE EXTENSION line hasn't been run.