Sure it can be done. For example, Hobbes' UFO Redux mod contains many new terrains, both for terror missions and the wilderness - though I think urban areas are just for terrors there. You can easily adapt them for landing and crash sites though.
In order to make new terrain, you need two programs. The first one is MCDEdit, which allows you to make tilesets - collection of tiles, containing their appearance (a graphic file, or several files if the tile is animated) and various settings (whether you can walk on it, how durable it is, how likely to burn from fire, etc.). MCDEdit does all that and is pretty easy. The other is called MapEdit and it's specifically to make actual mapblocks (pieces of terrain that the battle map is made of, usually 10x10 or 20x20 tiles) using the tilesets - it's a bit more tricky to use, but nothing hard. If you only want to make new map blocks using tilesets you already have, for example make more Forest maps or City maps from the same resources, you only need MapEdit.
If you're interested in it, try it - I guarantee it's not beyond your abilities if you wish to spend time on it (building is always time consuming, and this is a bit like Lego). There are several experienced mapmakers here, so we'll help.
