It isn't specular value here, it's ambient.
Yes, it's possible. Select your object in the world browser, and in the object manager, aspect tab, right click the preview sphere of the first material of your object and choose "edit all materials", then change the diffuse/ambient proportions. It will be applied to all materials.
But Vincent is right, it is better to leave these settings as they are, and change parameters in the atmosphere editor. Unless there is only one "trouble" obejct in the scene, and the radiosity settings are fine for all other objects.
This is done in Vue Infinite, I don't know your version, and I don't know if this can be done in all Vue versions.