That's a difficult question to answer without foreknowledge on the piece itself.

The more flexible the transition piece, the lower the loads will be.

If the manufacturer cannot give you ballpark answers on the "normal" stiffness of these transition pieces, I would be inclined to place a cnode anchor with 5 or 6 different stiffness levels and provide them Fx/y/z and Mx/y/z as a function of those stiffness levels.

Expect to revisit the analysis when they provide you a design for approval.