In general I agree with Borzki. However, I do not see CAESAR calculated natural frequencies to be a viable input to the EIG, due to relative conservatism of CAESAR versus EIG, if referring to the EIG pipe flexibility classification table.

Although, if you're in a situation where you need something, and something is better than nothing, and using natural frequencies from CAESAR for the EIG are resulting in passing designs despite the conservatism, I can't find fault with something that works.