Since I could not find any more info on the inbuilt function, I have written a function to do it using 2 API functions.
EMBED : Inside the GLOBAL Map
MODULE('api')
lstrlenW(long lpWString),signed,proc,pascal
WideCharToMultibyte(ulong,ulong,Long,long,long,Long,long,long),bool,pascal,raw,proc
END
If inpBSTR is the BSTRING and outCSTR is the CSTRING to be returned :
ConvertBSTRINGtoCSTRING(LONG inpBSTR, *CSTRING outCSTR),BYTE
ReqLen LONG,AUTO
CharWritten LONG,AUTO
CODE
CLEAR(outCSTR)
ReqLen = lstrlenw(inpBSTR)
IF ~ReqLen THEN RETURN(False).
IF SIZE(outCSTR)-1 < ReqLen THEN RETURN(False).
CharWritten = WideCharToMultiByte(0, 0, inpBSTR, ReqLen, ADDRESS(outCSTR), SIZE(outCSTR)-1, 0, 0)
IF ~CharWritten THEN RETURN(False).
outCSTR[Charwritten+1] = '<0>'
RETURN(True)