How to ignore a table/class in EF 4.3 migrations

The correct workflow in this case is creating first migration prior to adding changes (new classes), than adding new classes and after that creating new migration where you will have only new tables.

If you didn't use migrations so far the framework will generate migrations for all tables you have in the project because it believes you are creating initial migration. Once you have migration generated you can modify its source file and remove CreateTable code for old classes from Up method. The problem is you will probably have to do this in any subsequent migration.

Edit: I wrote a walkthrough for adding migrations to existing project with EF 4.3.1

With EF 4.3.1 released there is built in support for this scenario. When adding classes that are mapped to existing tables in the database, use the -IgnoreChanges switch to Add-Migration.

This will generate an empty migration, with an updated meta-data signature that contains the newly added classes.

Usually this is done when starting using EF Migrations, hence the "InitialMigration" name:

Add-Migration InitialMigration –IgnoreChanges