I suggest you contact the vendor and ask them how this type of joint should be modeled in a (beam element) piping program.

CAESAR II uses a 3D beam element formulation. This means that everything is modeled as an infinitely thin stick, with properties. Expansion joints fall into this category also. However, instead of the software determining the elemental stiffness properties, the user defines these values for the bellows element. EJMA rules and standards are only (optionally) employed at the back end, after the analysis, if the user desires to check/verify the expansion joint.