Hi! I have used the (m*mbo) component com_importusers
see
forum.joomla.org/index.php/topic,1015.0.html for discussions. I modded this to add additional fields at the same time eg postcode as password, all in another postal mail-out database. This is of course using standard mod_login basic fields, and deals with jos_core_acl_aro and jos_core_acl_groups_aro_map tables as well.
I am shortly going to use a series of passes to add much more data from the same database to CB, by
1. exporting the jos_users id and username having had mambo/joomla allocate the id
2. join the id to the external database by username
3. insert the additional data to CB fields, using perhaps MaxDB or writing INSERT statements using a reportwriter in external database. I will report back on success / problems!
Trevor