a=1:10
b=31:40
c=data.frame(a=a,b=b)

# create data in R
a = 1:10
b = 31:40
c0 = data.frame(a=a,b=b)

#  write data object 'c0' to existing Excel file
# 'Book1.xlsx' into a new sheet called 'Sheet1'
library(XLConnect)
writeWorksheetToFile(file = "C:/.../Book1.xlsx", data = c0, sheet = "Sheet1")