Pierangelo,

Has the status of back_sql as a valid syncrepl provider changed since this conversation:
http://www.openldap.org/lists/openldap-technical/200904/msg00145.html

I have a back_sql producer that I want to query and populate/synchronize to a back_hdb consumer.  From the 2009 email exchange, it sounds as though it is impossible or unsupported, though some emails from 2004 made it sound as though its feasible.
http://www.openldap.org/lists/openldap-devel/200410/msg00020.html

--------------------------------------------------------------
# My SQL Producer

database null
suffix "cn=admin"
rootdn "cn=admin"
rootpw password

#Clone of SQL entries for verifying syncrepl configuration
#database hdb
#suffix "ou=virtual"
#rootdn "cn=admin"
#directory /var/lib/ldap/virtual

database sql
lastmod on
readonly on
suffix "ou=virtual"
dbname dbname
dbuser dbuser
upper_func UPPER
use_subtree_shortcut  yes
children_cond    "ldap_entries.dn=UPPER(?)"
subtree_cond     "UPPER(ldap_entries.dn) LIKE UPPER(CONCAT('%',?))"
has_ldapinfo_dn_ru  no

overlay syncprov
--------------------------------------------------------------
# My HDB Consumer

database null
suffix "cn=admin"
rootdn "cn=admin"
rootpw password

database hdb
rootdn "cn=admin"
directory /var/lib/ldap

syncrepl rid=123
        provider=ldaps://localhost:636
        type=refreshOnly
        interval=00:00:05:00
        searchbase="ou=virtual"
        schemachecking=off
        bindmethod=simple
        binddn="cn=admin"
        credentials=password
        tls_cacertdir=/etc/pki/cacerts
----------------------------------------------------------
# Producer logging from -d 16384
syncprov_search_response: cookie=rid=123,csn=20130724213107.411609Z#000000#000#000000
slap_queue_csn: queing 0x7fe7fbffcb70 20130724213725.653540Z#000000#000#000000
slap_graduate_commit_csn: removing 0x7fe7f4168ef0 20130724213725.653540Z#000000#000#000000

-----------------------------------------------------------
# Consumer logging from -d 16384
do_syncrep2: rid=123 LDAP_RES_SEARCH_RESULT
do_syncrep2: rid=123 cookie=rid=123,csn=20130724213107.411609Z#000000#000#000000
slap_queue_csn: queing 0x7fba781ae900 20130724213107.411609Z#000000#000#000000
slap_graduate_commit_csn: removing 0x7fba781ae880 20130724213107.411609Z#000000#000#000000
syncrepl_updateCookie: rid=123 be_modify failed (32)

--
Raymond Page