Thanks for your comments, Rudy.
'Phase' migration is not an option for me as those 30 dbs host one giant app. So very much all or nothing. I've talked to Microsoft SSMA engineer and learned that most likely they will include this feature 'continuous sync-up data between Sybase and SQL Server' in future version of SSMA since they've received quite a lot requests from big customers to add this capability. Anyways, in the meantime, per SSMA engineer, the only Microsoft solution is to use SSIS or t-sql MERGE to merge the data table by table. I guess...spending time to implement this custom solution and only use 'one-time' is not any DBA's favorite :-)