Usually you can, but you always (alwaysalwaysalways) have to keep in mind the safe usage rate for both of them, and calculate how much of each oil can go in the batter. You can not just use the max of a fragrance (i.e 5% and then randomly add % essential oil on top of that)
So be diligent with the calculations, deduct a % of the fragrance oil and find out how much of the essential oil you can add without danger. So read up on safety issues for the specific essential oil you want to add. Some essential oils are sensitizers, and/or have to be used only in VERY small dosages.
And also, if you mix both, even if both of them usually are wellbehaved in soap, the mix of the two can make the batter trace very fast. So just be prepared. But the most important thing is usage rate for each oil.
eta Sorry, did not see Shunt already had answered, my internet connection is a bit slow so the refreshbutton didn`t work fast enough.