Everything in the Dromed world you build must be within the coordinates 1000,1000,1000 to -1000,-1000,-1000.
- Make an area brush surrounding the whole mission or part you want to move.
- Use the filters and make it so only the objects are showing - delete any complex objects with particle effects (like torches, magic stuff, whatever). If you skip this part you may double all the objects in your mission which is very problematic.
- With the area brush selected, press "multibrush me"
- Save the multibrush somewhere
- Load the other mission
- Load the multibrush you just saved, and place the multibrush where you want
- click on some other brush not in the multibrush to dissolve the multibrush
- Highlight Unsnapped
- Snap Highlighted
- Clear Highlights
(Those three steps are easiest when using custom menus)
- SAVE!!!
- Optimize

If it crashes, try moving things around, and check to make sure no part of the mission has coordinates outside the "legal" Dromed limit! Some of your previous links will be okay, but some or most will be screwed - so check them all.