Depends on what feature (tile) you are talking about and what tileset you want to add it to. At the very least it will require editing the SET file so that the tile appears - is defined under [TILES]. If the tile is a feature or part of a group, you'll also need to define it under [GROUPS]. If part of a group, you'll also need to add the other tiles that go along with it. Then you'll need to define the feature or group in the ITP file for the tileset.
You'll also need to make sure that the tile is the same height as the rest of the tileset and that the walkmesh corresponds with what surface materials are used in the tileset. For example, a long time ago someone added a tile from a desert tileset to the Rural tileset, but forgot to change the basic ground material in the walkmesh from sand to grass - although they had retextured the sand to appear as grass. Thus, you were walking over flat grass on that tile that reacted like sand and had no 3D grass appearing.
That being said, you might have to retexture the tile so that it meshes better with the new tileset. Nothing looks worse than a tile that stands out because the texture pallette is totally different than what was used in the rest of the tileset.
