Open up the map you want to copy the terrain too.
Then modify the tileset to match the tileset of the map you are copying from.
Save as (do not close it) whatever you want to call it. go advanced, view all the map.
Now open up the map you want to copy the terrain from. Go advanced, view all the map.
Select the terrain tool pallet, hit the space bar to deselect all tools. Now you will be able to draw a square (it will be highlighted green). You can select all of the terrain by holding down control and hitting A.
to copy hold control and hit c. to paste to your new map hold control and hit v.
The pasting process can take a long time, it will say 'pasting objects' in the upper right hand corner of the screen.
If you want only a piece of a map I would suggest hitting G and viewing in the grid format.
An interesting side effect of copying from one map to another is that you will have the option to rotate and mirror the bit of terrain you are copying.
Paste the terrain and use menu bar Edit functions (do not hit space or select another tool, leave the copied bit highlighted to turn/mirror it.)
Mind, you must have the same terrain tile set in the same order. If you do not have that set up then the computer will switch the tiles as per their order. This can lead to some interesting effects - not always wanted effects.