Yes, an entry is required in the DBCON table.
As the source system is running on MSSQL you need to make sure the DBSL files for Sybase is also present. I believe it should be there as newer releases of kernel comes with the DBSL for all supported databases. You also need to install the Sybase client software and adapt the environment variables. The steps are documented in these SAP notes.
1737415 - Sybase IQ: Enable remote/secondary connect to SAP Sybase IQ
1507573 - External DB connect to a Sybase ASE database
Cheers
RB