I've had some comments related to my previous post. Specifically they relate to importing alignments, making some edits and re-exporting them again.
As I said in my earlier post, this is not a workflow that the extension was designed to support, but since the question seems increasingly common I thought I would further detail the rationale here.
Because of differences in support and logic in the way alignments are calculated the only way Civil 3D can guarantee to accurately represent alignments from GENIO is to create all elements, including spirals as 'fixed' elements. If we didn't follow that route then we would be certain to at least change the geometry slightly and if we looked to calculate tangency from the geometry we may calculate a substantially different alignment. It's also possible we not be able to resolve tangency and have to reject the alignment completely.
Note that neither GENIO, nor the MX modelfile store the logic, nor input data by which the alignment was created.
Because the result in Civil 3D is an alignment with all elements fixed, you cannot re-export many of these alignment to MX, because MX has no concept of a fixed spiral. It is however possible for a Civil 3D (2012 and later) user to edit the alignment element fixity and recalculate the curve, spiral tangency in order to export back to MX.