A baseline migration of the current items must be established. So, the following should be the initial migration you create:
add-migration Initial -IgnoreChanges
By selecting ignore changes, EF is instructed to only save a snapshot of the live database. Now, the current tables won't be included in any future migrations (just the changes). Since EF will see the record in __MigrationHistory or deploy to a new, empty database, this would enable you to keep updating your current database.
Look at the This is the part about the engine.