yes, you can model variable kitchen exhaust using designbuilder. you could do this in a couple of ways.

you could create a kitchen zone and add a zone exhaust fan. within the zone Exhaust fan dialog, check "apply flow fraction schedule", then apply the fractional schedule to simulate the variable speed kitchen hood exhaust. I use a schedule that ramps up and down during peak catering loads.

Now, if you have a make up air unit, you will need to connect that and try to get the flows to work out. I have had trouble with this.

Here is how I do it: Do not use zone exhaust fan.

Create a separate zone (using virtual partitions) in the kitchen for the kitchen hood and include the catering equipment loads.

Add a generic air loop and configure with the desired components (htg and clg coil, etc..), but you will want to hard size everything, no autosize. add a VAV no reheat ADU to the zone. hard size the maximum air flow rate. select scheduled zone minimum air flow method. Use the same kitchen exhaust schedule here.

here comes the part where you have to trick EnergyPlus. Since the VAV no Reheat ADU modulates the airflow base on zone load, you have to set the zone heating and cooling setpoints way out (20F for heating, and 120F for cooling). This way, the ADU will always be at minimum airflow and use the schedule. This is why I create the separate kitchen hood zone within the conditioned kitchen.

let me know how it goes! -Greg