You are missing one force on the FBD you just drew. And instead of F normal of the system, I suggest that you think it as F12 (i.e. force of mass 1 onto mass 2) Also the friction force on the FBD of your initial post should be pointing in the other direction.
Let's say you slowly and steadily placed the object onto the spring, how much will it compress? This will be your new equilibrium point for the spring-object system.
Homework Statement
A uniform cylinder of radius R and mass 8kg has an off-axis hole drilled through it at 2R/5. Its new mass is 6.5kg. The hole and cylinder are parallel with their centers being the same height. What horizontal force, F, must be applied on the top to keep the cylinder at rest...