You can reflect this by adjusting your workforce and activity so that they are directly relational to the level of absence, i.e. you could upload your workforce with adjusted WTE against the roles that are producing less activity due to sickness.  If you then chose to model an increase in workforce to the actual WTE value, the tool would show you how much that sickness is costing or how much additional activity could be done if you didn’t have the high sickness rates.
If you are in any doubt as to how to include this in your model, then please contact the WRaPT team for further support.