Answer:
The direct input of a spring constant of the surface foundation is possible with a maximum of 2*108 kN/m3.
However, you can edit the stiffnesses of your surface supports in your load cases or even load combinations. If you multiply the value of the spring constant by a factor of 5, you can also see a stiffer support in the results.
Pay attention to the control of the stiffness modification, as explained in FAQ 4212.