The boxplot function in R uses a low-level function called bxp which accepts summary statistics. A simple example (lower whisker=1, 1st quartile=2, median=3, 3rd quartile=4, upper whisker=5) would look like this:
summarydata<-list(stats=matrix(c(1,2,3,4,5),5,1), n=10)
bxp(summarydata)
If you want to know more about the data structure that bxp accepts as input, look at the return value of the high-level boxplot function for some dummy data, i.e. try
sd<-boxplot(dummydata)
str(sd)
