If you are already qualified, then yes, it is pretty easy. For the CAS, you need to complete the professionalism course (COP), and they will grant you FCAS status. The SOA has a similar scheme.
If you are a student, then I imagine that it may be more problematic. I think that it would be difficult to get a US company to sponsor you to do UK exams.
Your best bet would be to try and get a job at a US office of a UK company.