Here is one solution:
library(openxlsx)
FILES <- c("demo.xlsx", "dummy.xlsx", "ExcelOut.xlsx")
FileList <- vector("list", length = length(FILES))
for (i in seq_along(FILES)) {
FileName <- paste0("c:/users/fjcc/Documents/R/Play/", FILES[i])
FileList[[i]] <- read.xlsx(FileName)
}
write.xlsx(FileList, "c:/users/fjcc/Documents/R/Play/MultiSheet.xlsx")