I can't speak for all EJ manufacturers, but they "should" be providing you stiffness values in units force per unit length and unit moment per unit rotation.

They "should" also be providing you permissible forces or permissible displacements.

If they aren't, I cannot speak on their behalf and promise you that they're going to approve anything you send them.

That's between you, the person buying the equipment, the sales representative of the EJ, and the engineer responsible for the EJ to agree how to proceed.

Edit to add:

However, if your expansion joint stiffness is significantly lower than the rest of the unit, you can expect the loads on the unit itself to be fairly low - almost as though it's cantilevered and not attached to anything if their stiffness is 0.

At least, that's the intent.

How well real life reflects that intent (and how it should therefore be modeled) can be debated.