The easiest way is to give an idea of desired file structure:
each backed up client is a subdirectory of /bak
e.g.
/bak/client1/
/bak/client2/
then within each client subdirectory Bacula Storage server should automatically create Volumes with format: $date_$time_$backup_type (one volume per backup run).
e.g.
/bak/client1/20120104_1654_full
what configuration could achieve this?