"Can we "insert" two more layers between existing layers?" Yes:  Your post implies that you want to convert a 2 layer board into a 4 layer but then you say that: "We currently have two ground plane layers." which is confusing if you are starting with only a 2 layer board. Without sight of your (private) project or a sketch of your new layer stack up, I'm not sure what you mean but if you: "Put USB traces on upper and leave lower blank ( save for vias) for uninterrupted ground plane." meaning on the two new inserted layers, then you must be careful to not create crosstalk between this new USB signal layer and whichever of the existing outer layers is above (or below) the new USB layer.