It is possible to design plenums in drawing.tool for use in your models. However, this may not always be necessary, depending on the project type.
The closed geometry used by the EnergyPlus engine requires that zones be defined from floor to floor, as this removes any potential gaps or slivers.
This means that if the floor-to-ceiling height is 10 feet, and the plenum space above the ceiling is 3 feet, then the floor-to-floor height to be modeled will be 13 feet. This approach will include the additional envelope loads from the portion of walls that border the ceiling plenum.
When assigning airflow rates by the ASHRAE Standard 62.1 Ventilation Rate Procedure method, this is also a good approach as all the inputs are based on area, not on volume.
For some project types, such as labs or hospitals, the volume of each room plays significant role. For these, you can add an elevation representing the ceiling plenum with its own walls, floors, etc. defined, along with its own room. In the image below, the plenum spaces are highlighted in blue and are modeled at an elevation considered to be the ceiling plane. All plenums are assumed to be unconditioned and unoccupied, so they should be assigned a Void Room Template. Additionally, the floor of these spaces is acting as the ceiling of the space below, and the ceiling of the occupied spaces is acting as the floor of the plenum. Thus, it's important to match the plenum's Floor U-Value and the occupied space's Ceiling U-Value to that of the actual ceiling material to be installed.
Depending on the needs of your project, there are two methods to model the plenum. The first is as shown above, where each occupied zone gets its own plenum, both of which are grouped up together in the same zone.
The second is as shown below, which is where the plenum of every room is joined together as one to form one large plenum zone that sits atop the divided rooms below.