By default, the migration process assumes the target WordPress site is a fresh installation and removes all existing content before adding the new one (by “content” we mean are pages, posts, comments, etc, as all configuration tables are respected). This is necessary to prevent the new data to clash with the existing ones (which, for instance, could cause errors to duplicate identifiers).
Nevertheless, the migration can be adapted to protect your existing WordPress content. We are able to take as input, apart from the site-to-be-migrated, your current WordPress site and make sure that the converted data is added on top of the existing one without affecting it.
Posted in: Features Migrating your CMS on top of an existing WordPress site