In order to do this your going to need to save both maps as editor component lists. Now you should have two folders where you saved the maps. (named what you save them as.) Open the one that has the terrain and copy the following files. Minimap.tga, Objects, Objects.version, and Anything that says t3 as a prefix. Paste and replace those files in the folder with the data and triggers. Now open the data and triggers component's list in th editor. You should now have the terrain. Save as a normal map file.

All credit goes to Kerensky he came up with this idea