In Ram Structural System - Ram Modeler the user can import dxf files, but the process is a little different than it is in Ram Concept. In RAM SS, imported cad objects are directly converted into Beams, Columns, grids etc. See the Importing DXF Files into RAM SS wiki for details. Since version 14.03.00.00, it is possible to import a dxf file onto a new layout and then reference that layout for the purpose of aiding the modeling of another layout. There are numerous possibilities for this feature, one workflow is illustrated below.
Start by creating a new layout and import the prepared CAD file which has the changes that need to be made to the RAM SS Model.
When importing, select the desired layers. Importing CAD circles as RAM Columns and CAD lines as RAM Slab edges are the two easiest ways to set up a snappable reference objects in my opinion.
Here's an imported layout with only columns and slab edges.
Now switch to the existing floor plan and use Options - Reference Layouts to show the new CAD layer as a background.
Use Options - Set Snap points to make the objects of the reference layout snap points.
Now use commands like Move Columns, Move Walls, Move Beams, Change Deck Polygon etc. to revise the existing layout based on the new snap points.
Once done, repeat for other layouts or use Integrity - Align Columns (and Align Walls) to coordinate the other stories.
The Cad Background reference layout can be purged from the model at any time using Layout - Type - Select - Delete.
Importing DXF Files into RAM SS
RAMSS Modeling FAQ