Member definable stiffnesses represent a special 'Member Type' (see the Members chapter). Thus, it is not necessary to define any cross-section, but you can directly define the elements of the stiffness matrix.
Enter the values for the individual 'Stiffnesses' and define the 'Parameters for Self-Weight', the position of the 'Main Axes', and the 'Coefficient of Thermal Expansion', including cross-section dimensions.