As already explained, there is no specific support for doing things like that in ATENA. I can only recommend to save a simple ATENA 2D model as .cct and look at the file structure. Some users are generating (parts of) this file by various tools (spreadsheets, self-developed SW, ...). In you case, you would have to define all the joint coordinates, lines, and macroelements (as lists of boudary lines) and make sure your macroelements are well interconnected (i.e., each line except the outside borders belongs to 2 macroelements) and no MEs are overlapping. You would also have to pay attention to the supports and loads (or assign them manually after importing your .cct into ATENA 2D).

Another option is to use the "MaterialWithRandomFields" by manually editing the ATENA .inp file (e.g., stored by ATENA-GiD from a model prepared in GiD) and prepare the spatial distribution file by hand, respectively write your own program to create the file. Before trying that, I definitely recommend to ask our sales colleagues to prepare an offer for SARA - you can ask for 2 variants, to buy it or to just rent it for the time of your project.