R help - Dec 2010 - adding more columns in big.matrix object of bigmemory package

Hi all,
   Is there any way I can add more columns to an existing filebacked big.matrix
   object.
   In general, I want a way to modify an existing big.matrix object, i.e., add
   rows/columns, rename colnames, etc.
   I tried the following:
   > library(bigmemory)
   > x   
read.big.matrix("test.csv",header=T,type="double",shared=T,backingfile="test
   .backup",descriptorfile="test.desc")
   > x[,"v4"] = "new"
   Error in mmap(j, colnames(x)) :
     Couldn't find a match to one of the arguments.
   (The above functionality is presently there in usual data.frames in R.)
   Thanks in advance,
   Utkarsh