Here a solution using appendWorksheet
from XLConnect
package:
library(XLConnect)
# mtcars xlsx file from demoFiles subfolder of package XLConnect
demoExcelFile <- system.file("demoFiles/mtcars.xlsx", package = "XLConnect")
wb <- loadWorkbook(demoExcelFile)
appendWorksheet(wb, mtcars, sheet = "mtcars")
saveWorkbook(wb,demoExcelFile)