A Sony spokesperson has confirmed that PlayStation 5 users will not be able to install their own NVMe SSDs on the console at launch. Users may have to wait for a few months for Sony to enable the feature via a software update. However, the console will support external USB hard disks, allowing users to play last-generation titles with ease.
https://www.notebookcheck.net/The-PlayStation-5-will-not-allow-users-to-add-more-solid-state-storage-at-launch-functionality-expected-to-arrive-via-a-future-update.502361.0.html