A level map is just a model just like a character or a ship in your game. You can use the XSI Mod Tool to create the map just like any other model in your game. The XSI Mod Tool does limit the number of triangles you can have in a single model, but you want to keep the number of triangles as low as possible anyway so that the scene can be rendered at a reasonable number of frames per second. If necessary, you could break your map into several models. If you want a full featured modeling tool that has no limitations, you could get a full version of Softimage XSI from Avid or perhaps Maya or 3D Studio Max from Autodesk. However, these programs are quite expensive.
