Hi Lucas,
the examples below give you an idea how to add a column to an ffdf.
library(ff)
# physically single columns
x<-ff(runif(10,0,1))
y<-ff(runif(10,-1,1))
z<-ff(runif(10,2,3))
fdata<-ffdf(x,y,z)
a <- clone(x)
fdata2 <- do.call("ffdf", c(physical(fdata), list(a=a)))
# physically combined columns
x<-ff(runif(10,0,1))
y<-as.ff(cbind(y=runif(10,-1,1), z=runif(10,2,3)))
fdata<-ffdf(x,y)
a <- clone(x)
fdata2 <- do.call("ffdf", c(physical(fdata), list(a=a)))
colnames(fdata2)[1:ncol(fdata)] <- colnames(fdata)
HTH
Jens