Hi Piticli,
Welcome to the Mattress Forum! 
Measuring ILD accurately in visco-elastic memory foam is problematic, as this can vary dramatically by the foam’s rate sensitivity (recovery rate after compression), temperature sensitivity and humidity sensitivity. Sample conditions need to be highly standardized. It’s safest to state that the memory foam commonly used for comfort layers in a mattress are all quite plush, and generally in the upper single-digit to the upper teen ILD range.
While your mattress may not feel “very soft” to you, realize that all of the layers of a mattress work together to achieve comfort, so the firmness you feel will be mostly from the deeper layers of polyfoam.
If you’re considering adding 2" of memory foam to your mattress, you would be increasing the surface plushness, and overall the mattress would feel a bit softer.
If you haven’t had your Novosbed for very long, you may wish to test it out a bit longer to see if it softens up a bit more to the range of comfort that you may desire. If you have indeed had it for a bit longer, then adding a topper certainly is a popular method to achieve a bit more surface plushness.
If you’re interested, there is more information about choosing a topper in post #2 here and the topper guidelines it links to, which can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success.
Phoenix