The answer is technically yes, you can put a cheater bar on a torque wrench. However, doing so defeats the purpose of using a torque wrench in the first place. A torque wrench is designed to help you apply a specific amount of force to an object - usually for the purpose of tightening or loosening bolts. By attaching a cheater bar to the end of the torque wrench, you are essentially increasing the amount of force that can be applied - which means that it's very easy to over-tighten or break the bolt entirely.
There are some situations where using a cheater bar may be necessary - such as when working with particularly stubborn bolts. However, if at all possible, it's best to avoid using one.