I doubt there's a one-size-fits-all description to cover all scenarios. I'd suggest posting documentation what yours looks like, with permissive/restritive loads or movements with stiffnesses defined.

With that said, such an expansion joint would generally be modeled like any other expansion joint, except perhaps:

1. You might model the end connected by a restraint-cnode to pipe that features x, y, z, rx, ry, rz,
2. You may have limits on dx, dy, dz, drx, dry, drz OR fx, fy, dz, mx, my, mz OR both, which may be anisotropic in nature.
3. Slots may mean you also have anisotropic gaps in your x, y, z, rx, ry, rz restraints.