By default, the migration process assumes the target WordPress site is a fresh installation and removes all existing content (by “content” we mean pages, posts, comments,… configuration tables are respected) before adding the new one. 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 makes 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