The following video shows how to convert an imported CAD profile into a 3D alignment for a project. If you need the alignment to be a linestring, use the Convert to linestring command at the end of the process to make that conversion.
The video shows how to Copy the Profile into the profile view for an alignment.
How to scale the vertical profile
How to move the profile to the correct station and elevation
How to combine the alignment and profile to create a 3D alignment
Video shows you how