Yes, as Jeff says, applying any external voltage directly to the motor while it is also attached to the orange and gray decoder wires will most likely damage the decoder.
But why are you even considering this? If the analog mode is enabled on the decoder (it is in most cased a factory default), then applying DC voltage to the rail pickup (wheels) will make the motor run. That is how I very often test and clean my decoder-equipped locos. All decoders sold nowadays should be capable of running from DC (analog) voltage. Even all the older decoders I have work with had that capability. I think that is part of the NMRA standard. Some model companies tout that their decoders are "dual mode", while in fact all the decoders have that capability.