I would not expect that the approximation would introduce an offset of ~600km.
What I think happens here, is that the exported geometries have ended up with coordinates having a different origin than the origin of the coordinate system itself. Depending on which product is used when exporting, this can sometimes be default behaviour, and other times be configurable behaviour. Forma, by default, expects coordinates to have the coordinate system's origin as origin.
When doing the export, do you see any options that would allow you to set what the origin for the export should be? At times, products rely on local coordinates internally, relative to some defined reference point. I think that here, this reference point is ~600 km off the coordinate system origin, and that this is what introduces the offset when importing the data.
You could try to export data using the coordinate system's origin as the data origin, or, you could use Forma's built-in support to compensate for such offsets. If the offset values are known (shown in Civil 3D) you could manually add these values in the dialog shown in the screenshot you provided, in the X / East, Y / North fields, and then run the import. That should make things align again (pending any offsets that would be introduced by the approximation).
Best regards,
Kristoffer
