Answer:
A block is initially saved as in RFEM 5 / RSTAB 8, and imported via Dlubal Center:
1) Select the objects to be saved.
2) Menu "File", "Save as Block", please note: Enter the model category; see Image 01.
3) In the target file, open Dlubal Center via the "File" menu or the toolbar and open the block in the corresponding model category; see Image 02.