3rd partyのバックアップソフトのイメージからのVM変換は Converter 6.0.x が最終となり、6.1.x以降は機能が削除されました。
6.0.x をダウンロードして変換が可能か試す必要がありますが、互換性のなどの確認も必要となります。
VMware vCenter Converter Standalone 6.0 Release Notes
VMware vCenter Converter Standalone Release Notes
VMware vCenter Converter Standalone 6.0 is the last release of the product to support third-party backup images and virtual machines as sources for conversion. This capability will be discontinued in the next release. If you use this capability, you should start planning your transition.
For the full list of the third-party backup images and virtual machines see Interoperability
.