well BSP is a compiled format not a source format...
infact it was designed quite specifically for speed and not to be loaded or saved as a standard format. Converting something to BSP should be the end of its editing life, and it is quite a bugger to actually decompile them as it were.
thats what the .map format is for
and there is another convert which decompiles most BSP formats to thier original .map format - however again there is the problem that Max doesn't contain an importer/exporter ... mainly because of licence issues, but also because you ever tried to work with the Max SDK - it is a bitch, and thats putting it nicely. It's far easier to just use the purpose built BSP editors.
however most of the BSP->whatever format also come with full source, so really all you'd need to do is edit the BSP format data header - setup the new routines of howto handle certain part, like the Spline Patches ... all of which can be found within the Quake3 Tools SDK 1.27h
it is alot more bother than it is worth to be honest, so unless there is a REALLY and i mean REALLY good reason you need A particular BSP world - then i'd suggest just working on a world in Worldcraft, export as DXF and then use Max to texture and Export as a DirectX (^_^)
Tsu'va Oni Ni Jyuuko Fiori Sei Tau!
One block follows the suit ... the whole suit of blocks is the path ... what have you found?