SYSAUX tablespace is 98%
No, Oracle won't add an extra datafile.

Normally the SYSAUX tablespace is more or less stable so it would be smart to check what is eating the space in there. Connected as a DBA user, run the script ${ORACLE_HOME}/rdbms/admin/utlsyxsz to get the current usage of the SYSAUX tablespace and see how it will grow when you change certain parameters for which you are asked to enter values.

See Oracle® Database Administrator's Guide 11g to get an idea of regular sizing.