No, you can create levels using any modelling application that can export any model format supported by DarkBASIC Professional, although .X is a popular format.
Check out the 3D modeller programs available from the sticky thread of 3 Dimensional Chat. I would also recommend Cartography Shop. A demo can be downloaded from the World Building menu at the top of the page.
BSP is ideally used for indoor locations and can be created using Valve Hammer. A thread has also been created to deal with BSP issues, located here -
http://www.thegamecreators.com/?m=forum_view&t=23443&b=3 - 'The Ultimate BSP Thread'. It details links to obtain the necessary tools if you need them.
Because DB's collision system isn't suitably implemented when dealing with .X geometry, I would also suggest that you take a look at Nuclear Glory's collision plugin system for DarkBASIC Professional which can handle large .X map files -
http://www.nuclearglory.com
Hope this helps.
Paul.