j'ai trouvé:
> res <- as.data.frame(t(c(A=1,B=2,C=3)))
> res
A B C
1 1 2 3
> write.table(res, file="file.csv", sep="\t", row.names=F, col.names=T)
> write.table(t(4:6), append=T, file="file.csv", sep="\t", row.names=F, col.names=F)
> read.csv("file.csv", sep="\t" )
A B C
1 1 2 3
2 4 5 6