you have the XLConnect package. Read the documentation or the vignette of that package carefully to know exactly what is possible.
Using XLConnect you normally overwrite the cell styles unless you set the style action to be "none" using.
To set you on the right road, a trivial example :
wb <- loadWorkbook("test.xlsx", create=TRUE)
Data <- data.frame(
a = 1:10,
b = letters[1:10]