I explain the following case:
You have a drawing file (dwg1) with an alignment.
You copy this drawing file, so you have (dwg2).
You create a layout profile for this alignment in dwg1.
Now, from dwg1 you export this profile (you must also export the alignment) to a LandXML file.
You open dwg2 drawing and import this LandXML file.
Immediately, CIVIL 3D creates a new alignment and its profile but you CAN'T assign this profile to the existing alignment.
In my opinion, C3D should allow us managing the importation of objects contained in a LandXML if we want to create a new alignment and its profile or directly the profiles to an existing alignment in the drawing if it is possible, I mean, if the length of the alignment is at least the same length of the profile.
Maybe there are others objects where we could also stablish this.