Don't delete exist data with SSMA

I'm trying to use Microsoft's SQL Server Migration Assistant for Sybase 5.3 ('SSMA') to migrate multiple Sybase databases (that have the same schema) to a single MSSQL database. The first database migrated fine. But when I try to migrate data from the next one, I see lots of warnings like: The table 'MyDatabase.dbo.MyTable' contains data. If you continue with data migration, all data in the table will be deleted. I don't want the existing data to be deleted, but can't find a way to do this in SSMA.