@Maystogan,
You have "LAYMRG" command... When Dialog Box pops up, you select all layers to merge, then hit enter, then choose "N" - name to merge layers to - I choose here "0" layer when another Dialog Box pops up... That's all...
In your case after merging to "0" layer, you should create "XR" layer and merge "0" with "XR", or create "XR" layer and simply select "ALL" and change layer property of all entities from "0" to "XR" layer...