If you already have your games installed in Cemu but don't have the system menu, it means you likely only dumped the game files (RPX/RPL) and not the system architecture.
If you cannot access a real Wii U, you can download the menu files directly from Nintendo's Update Servers (NUS). Note: This requires obtaining decryption keys which are readily available online, but we will only point you toward the technical process. How To Get Wii U Menu On Cemu
If you have specific RPL files, place them in the appropriate system directory within mlc01 as instructed by the Cemu Wiki . Open Cemu. If you already have your games installed in
A homebrew tool used on the Wii U to dump the MLC (internal storage) and SLC (system NAND). How To Get Wii U Menu On Cemu