In order to open an RSTAB file in RFEM, you have to change the file type in the "Open" dialog box either to "File type RSTAB" or to "All Files". This way, you can display not only the RFEM files, but all RSTAB files stored in the selected directory. After opening the RSTAB file, you can save the model as an RFEM file for further editing.
However, please note that the printout reports or input data already created in add‑on modules, for example, cannot be transferred.