I'd suggest asking Kirkby Microwave (UK) - when I purchased one of their SMA cal kits some (3-4?) years ago, they had option (which I added to my order) for U.FL which ended up being a few adapters and a few short sections of 1.37mm coax, but by the time it got here the need at that point had passed, and I've not returned to U.FL-land so far since.
I believe Kirkby Microwave have iterated on their product since then, and their broad offerings include support well beyond 6 GHz, so they'd likely have device coefficients to support the connector in the frequency range you cite.