If you need to add an additional data file to an Oracle Tablespace:
ALTER TABLESPACE
users
ADD DATAFILE
'AdditionalUsersTablespace.dbf'
size 100m
AUTOEXTEND ON
NEXT 10M
More info from these posts:
http://www.dba-oracle.com/t_alter_tablespace_add_datafile.htm
http://docs.oracle.com/cd/B19306_01/server.102/b14200/statements_3002.htm
