Yes, the Xiaomi 15 Ultra supports UFS storage extension. The 512GB version supports an expansion of 16GB. The 1TB version supports an expansion of 32GB.
For your reference, here is the information of the UFS storage extension:
UFS (Universal Flash Storage) is a high-performance storage standard used in smartphones, tablets, and laptops, offering faster read/write speeds than eMMC. It includes advancements in both the physical interface and protocols, enhancing performance and efficiency. UFS enables simultaneous read and write operations through full-duplex data transmission, improving application loading, file transfer, and video recording. It also boasts better power efficiency and higher bandwidth, making it ideal for multitasking in demanding scenarios like HD video playback and mobile gaming.