If memory serves....
Go to Object Editor window ->File->Export->Save
Now open the map where will you import it then go to Object Editor window File->Open->Open your saved data previously
Sorry if have wrong terms about the steps because my Warcraft III on my PC got removed so maybe I forgot them already. That's all I can remember.
EDIT:
Enter the Object Editor, click File, and somewhere around the center of the list is an Export All Object Data button. You also find Export All Unit Data, Export All Doodad Data, etc.
Note, however, that importing new Object Data files erases your current ones (the Editor itself will warn you).