We have Callmanager 8.0. Cisco DirSync service is enabled and our LDAP source is Microsoft Active Directory. Is it possible to add a new LDAP field to the list of fields that are synchronized? For instance there is a column called building in the enduser table in the informix backend database, but its empty for all end users. We would like to use the physicalDeliveryOfficeName LDAP attribute to populate that.

UCM 9.0 introduced this capability via the 'addLdapSyncCustomField' request (corresponding UI in the UCM admin LDAP config pages.)  Prior to that, it is not possible to customize the fields for syncing.

Normally LDAP sync occurs on a schedule (every 1 to 7 days...at midnight..?)  It is possible to trigger an immediate LDAP sync from the UCM admin pages, or via AXL 'doLdapSync' request (UCM 8.0+)
Typically LDAP sync is configured because the number of users is large and unwieldy enough to require a dedicated directory system.  Therefore it is quite possible that a sync could have a performance impact, and doing so hourly (or indeed, during business hours) might need to be avoided.  Probably only careful testing (best done in vivo) can give you a complete understanding of the impact.
 
 

Not aware of anything immediately forthcoming, but my visibility is incomplete in that area.