reexpand a matrix after subsetting

Hi,

suppose I have a matrix (or dataframe) 
as a result from subsetting.

mat <- matrix(1:20,ncol=2)
mat[c(3,6,9),] <- NA
cc <- complete.cases(mat)
sub <- mat[cc,,drop=FALSE]
sub <- sub * 2
#some caluculations with sub.

now I would like to expand sub somehow
so row 3,6, and 9 would be filled with 
NAs but the rest should be in place again.
Is there a simple function for this?

merge is not an option.

Thank you very much for your help.