Sueposer,

Import both meshes into the Assembly room. Double click on one of the meshes to open it in the Vertex modeller (or select it from the instance folder). Once in the vertex modeller "Select All" and Copy. Go back to the Aeembly room and double click to open the other mesh. Once in the Vertex modeller with the second mesh "Paste" in the first mesh. Once it's pasted in you can go mack to the Assembly room and delete the previous instance of the mesh you copied.