Formula fields are special fields (virtual only!) which act on the script placed within them - ie they calculate in real time - Thus they don't store data. so you can't import directly into a formula field. If the formula relies of some action from one of more of your extisting fileds in your csv. Then you would simply create that formulal field to do the same action

ie in a sample csv table you have 3 fields of data costprice / sell price and profit - given that profit will always be sellprice minus costprice. Thus profit does not need to be stored it only needs to calculate in real (virtual) time.  You set up the 3 fillds in your table 2 to hold data the imported data (costprice and sellprice) and 1 formula field (profit) with the script selleprice-costprice