Yes. It is possible to offer both ICHRA and traditional health insurance. However, they need to be offered to different classes. You are not able to offer both plans to any single employee. Because part-time and full-time employees are different classes, you can offer one class, ICHRA, and the other a traditional group plan.
Suppose an employer offers an ICHRA to part-time employees and simultaneously offers traditional insurance to a different class. In that case, the employer must consider the minimum class size requirement (see minimum class size requirements for additional information).