Is it possible to set manually the cross section class?
No. You cannot set the class yourself.
The determination of the cross-sectional class is done automatically based on the Eurocode EN 1993-1-1 table 5.2. This is done for each ULS load combination and cross-section.
Section class 1+2: plastic, section class 3: elastic. Section class 4 automatically becomes section class 3 by reducing the yield strength fy. Cross-sectional classification is only relevant in the Ulitimate Limit State (ULS).