Answer:
If the PLATE‑BUCKLING add-on module is not opened as a stand-alone program, but using RFEM 5 or RSTAB 8, you can import the panels (c/t parts of a member cross-section) as well as the respective load cases of the current models in PLATE‑BUCKLING (see the image).
If no valid cross-sections of PLATE‑BUCKLING are found in the model file of RFEM 5 or RSTAB 8, the option for importing the buckling panels remains inactive.