Does using an antenna extension cable violate FCC cert?
The short answer is that should be fine.

I have not seen the FCC specifically address this exact question but I think it is fair to say you can infer their answer from their published guidance on permissive antenna changes.

In this document they talk about what type of antenna substitutions you can make without filing for a Class II permissive change. With an extension cable you are reducing the gain of the system but the antenna type is not changing and you should not otherwise be altering the in-band and out-of-band characteristics. 

You will still have to do FCC Part 15B unintentional radiator testing for the whole system the SOM plugs into (assuming you have at least some switching components on the carrier board for the SoM).

You probably won't find a u.FL to u.FL adapter cable, but you can find a u.FL to SMA adapters. 

You would still probably be better off just using their antenna builder and just getting the cable length you want.