Answer:
There is the simple option to orient new structural elements to the existing elements during the definition.
Find the "New Coordinate System" command in the drop-down menu "Tools" → "Coordinate System".
By clicking this button, it is possible to create a user-defined coordinate system to which the work planes are aligned. If you select three points that define the coordinate system in a way so that they are corner points of the existing surface, one plane of the new coordinate system is determined by the surface plane and the other two work planes are perpendicular to it.