Profiles extensions are stored in the profiles-config.xml file. See Adding custom extension attributes in the Customizing section of the IBM® Connections product documentation for more details on custom extensions.
Do not confuse custom fields with the default set of fields that are provided with a profile. That set of fields is predefined and the administrator can determine whether or not to let users edit them. For information about how to update the values of those fields, see Updating profile entries.