Of course it is not recommended, but people do it all the time. Keep in mind it may change the handling balance of your car, but I have seen people show up at autocrosses and drive at the limit with mismatched F & R tires. With the mad skillz of a Burl East, or a Lewis Wise, you can do anything!
If you don't push the car to the limits of adhesion, you may not even be able to notice the difference. In the wet or in an emergency manuever is no time to find out that your car is more tail-happy than it was, though!
Why don't you just replace them with more S03s? They are great tires, but a bit on the expensive side.
TT