The configuration supports setting the hash algorithm for user passwords to SHA-512. However SHA-512 hashes cannot be stored in the JPA user repository, because the field is too short.
http://svn.apache.org/repos/asf/james/server/trunk/data/data-jpa/src/main/java/org/apache/james/user/jpa/model/JPAUser.java sets the field length to 100:
/** Hashed password */
@Column(name = "PASSWORD", nullable = false, length = 100)
private String password;
The password hashes are hex encoded before they are stored to the database, so SHA-512 hashes take up (512/8)*2 = 128 characters.