Answer:
It is not possible to save RFEM files as RSTAB files. However, RFEM files can be opened directly in RSTAB. In RSTAB, set the file type to "RFEM" in the "Open" dialog box. The relevant RFEM file can then be selected and opened (see Image 01).
Furthermore, RFEM files can be opened using the Project Manager. To do this, open the "RFEM" tab and select the corresponding project. The relevant file can then be opened in RSTAB by selecting the "Open with - RSTAB X.XX" command in the shortcut menu (see Image 02).
Please note that not all functions of RFEM are available in RSTAB (for example, element type ARC, SPLINE, and so on). A message box appears informing you of these elements.