How to import Sybase .db file into SQL Server database?
Since both Sybase and SQLServer share the same SQL dialect (T-SQL), I'd generate the DDL statements from the existing database (it will require a Sybase server) to build the SQL Server database and the an SSIS task to copy data between databases.

I don't know if it is possible to SQLServer to just open a Sybase file, but, albeit the same origins, I don't believe there's any kind of binary compatibility between inner formats of this two DBMS.