http://scpcbgame.com/rooms.zip
These are in 3DW format. You need 3D World Studio to edit them. The demo version will not allow you to save your changes, so you need to "acquire" the full version.
You may want to follow my incomplete guide on mapping:
https://docs.google.com/document/d/1URs ... sp=sharing
(You can skip any steps that require source code editing if you're simply editing existing rooms)
The reason the maps in the game are in B3D format is because that is their compiled format, similar to models, and replacing/changing their textures wouldn't be very easy.