Not sure if it's the fastest, but it'll work if there are no NA
s in the data:
# just in case these aren't Dates.
NADayWiseOrders$date <- as.Date(NADayWiseOrders$date)
# all desired dates.
alldates <- data.table(date=seq.Date(min(NADayWiseOrders$date), max(NADayWiseOrders$date), by="day"))
# merge
dt <- merge(NADayWiseOrders, alldates, by="date", all=TRUE)
# now carry forward last observation (alternatively, set NA's to 0)
require(xts)
na.locf(dt)