As indicated by the previous poster’s response, the answer here in a little more complicated that one might expect.
There is nothing impermissible, per se, with assigning additional duties (including non-exempt duties) to an exempt employee. It is also OK, generally, to pay an exempt employee extra compensation for additonal duties, as long as the employee is still receiving his or her set salary.
An employer must be careful, however, because the exemption can be lost if an exempt employee performs too much non-exempt work. The old 80% limitation on non-exempt work no longer applies under federal law. The issue is whether the employee’s primary duties consist of exempt work… considered all relevant facotrs (frequncy and amount of exempt work vs. non-exempt work, the importance of the exempt work perfromed, etc). Adding too much non-exempt work into the equation can harm exempt status. Also note that some states (like Pennsylvania for instance) maintain a limitation on non-exempt work. So be careful to check state law.
Straying too far from the salary payment can also be risky. Additional compensation is OK generally speaking but the employer must be sure that the exempt employee’s salary is always paid on a regular basis less only permissible deduction.