try this (it seems that the keyword header is not the correct one)
I assumed you only want to write the header once.
using DataFrames
using CSV
df = DataFrame(A=1:3, B=4:6, C=7:9)
fi="c:\\temp\\append_a_line.csv"
fi="append_a_line.csv"
for i in 1:10 # do the following for 10 times
CSV.write(fi, df, writeheader = (i==1), append = true)
end