I faced the same challenge. I had setup a new grommunio server, but ldap was not working properly yet. (I had forgotten to import the self-signed root-ca of my UCS domain.) Impatient as I was, I created a local user and once ldap was working again, i wanted to link the local user to my ldap directory.
I used the brute force method of simply updating the mysql database:
mysql -uroot
use grommunio;
select * from users;
update users set externid = '<EXTERNAL_ID_FROM_LDAP_DIRECTORY' where username = '<YOUR_USER@YOUR_DOMAIN';
