Convert the BSP to a Map file.
Search on this Page www.thewall.de for the BSP2Map Converter.
Convert the map, check the Map for Broken or damaged Blocks, resave the Map, open it with the MED...
This could be done within less 5 minutes...

But the Texture WAD files are not same, like the 3DGS WAD files. So youll must create your own Textures and re-Texture the whole Map !

hope this helps a lot

cYa Sebastian

Tutorials:
[Blender]Terrain creation ENG/GER
[Blender]Low Poly Tree Modeling
[GIMP]Create a Texture for Terrains
CLICK HERE