In Workbench you can directly model your anti-roll tank, and the Aqwa solver will include its effect on the vessel RAO motions in the Hydrodynamic Diffraction calculation (any any subsequent Hydrodynamic Response analyses).
In the geometry editor you should add the tank geometry to the vessel, and split the surface bodies at the tank mean fluid level (the split is not necessary in Release 2022 R2 onwards). Then you will probably need to flip the surface normals, so that they point inwards, into the anti-roll tank fluid.
Once you have (re-)imported the geometry into the Aqwa Workbench editor, you should right-click on the vessel in the Outline tree and Add > Internal Tank. Select the anti-roll tank surfaces as the Internal Tank Surfaces, and select an edge at the position of the mean tank fluid level for the Fluid Level Definition (you can define the Surface Height directly in 2022 R2, and a Fluid Volume in 2023 R1). Finally define the Fluid Density; you can also set a Permeability (0.0 to 1.0 as a volume fraction) to account for any unmodelled internal features that reduce the tank volume. The Damping Factor can be used to dampen the motion of the tank fluid surface, if necessary.
I hope this helps!