If there is a way to add a projection method, I don't know how.
You might try opening a support request with Autodesk. Open it with the Map team, and not the Civil 3D team - I believe the Map team is the one that deals with coordinate systems.
You can sort-of transform a drawing. You can use Map to connect to other drawings, and then use Map Queries to pull the data from the source drawings into your current drawing. Map transforms items as it queries them.
Unfortunately, Map Queries do not work on Civil 3D entities. So you can query plain Autocad items, but not things like Alignments, Gradings, Feature Lines, etc.
You can also use LandXML to transform items between coordinate systems. So, if you have a Surface in one coordinate system, you can Export it to LandXML. As long as you have the coordinate system correctly set in your drawing before doing the export, the LandXML file will contain coordinate system information. Then, when you import the LandXML, it is reprojected to the coordinate system of the drawing you are importing into.
So you can kind of transform things, in a very stilted, cumbersome, and often-incomplete fashion.
-- Sinc
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Sinc