Might new bones evolve from soft tissue?
New bones do form occasionally so it is not impossible, Ornithischian dinosaurs are defined by a evolution of a new bone (predentary). There are many pieces of cartilage in the body that become ossified in some animals and not others, the kneecap is a great example since a bony kneecap has evolved and been lost many times independently, it is just a matter of ossifying existing tissue. The trunk however is more difficult since it does not have any suitable cartilage. 

Cartilage however is not necessary to form bone, but it is the most common way to get it. Endochondral ossification forms bone from cartilage and is responsible for the majority of the bones in the body. Intramembranous ossification forms bones in other connective tissue, the bones of the skull and dermal bones are examples of this process. Dermal bone has evolved separately many times but tends to form in the skin.

Given enough time  and the right selective pressures it is certainly possible for bones to evolve in the trunk but it is not going to happen quickly.