I didn't understand 100% of that, but here's my take:
Stepmania 3.9rc4 and Stepmania 4.0 support .BMS files, but there's no keysounded bms support. Shakesoda has been working on a theme (google beat man - THE THEME) for the past year which is for SM4 and supports keysounded bms files.
Also, there's this thread at bemani: http://www.bemanistyle.com/forum/f94/beatmania-add-4946/
And this file: http://www.megaupload.com/?d=4SDEQXXE
If for some reason your SM doesn't support bms files. Hope this helps.