There are two ways to merge meshes during export:
Any number of meshes of varying complexity can be merged. In this tutorial, we'll create create two boxes:
The blue box will be considered the main mesh, and the red box will be considered the one that needs to be merged into the main mesh.
To achieve this, the red box needs to be a child of the blue box. Select the red box and link it to the blue box using the Select and Link tool in 3DS Max:
Next, select the red box, and choose the Object Settings item in the OgreMax menu. Choose Submesh as the object type:
Now, when the scene is exported, the red box will be merged into the blue box as a single mesh. This can be seen by loading the exported mesh into the OgreMax Viewer and selecting it - there's one bounding box that encompasses the entire object: