There are two solutions I can recommend. The simple solution is to write it originally with the right formats. This is unfortunately not always very doable. The more complex solution is to create your own in memory editable representation of what you want the spreadsheet to be, and then write the whole thing in one go once you've got it ready.

In the past when I had to solve this problem I took the second solution. Unfortunately I can't show you that code because it is owned by a past employer. But if you're competent with OO code, it isn't that hard to create an OO representation of a spreadsheet...