Yes you can.
If you input data into your forecast that shows you spend £800 per month on rent, but for 2 months of the year, you get a discount, you can change this manually in the forecast to reflect this.
You need to go into your profit & loss forecast and click on the line item you wish to edit - in this case: rent.
Click on 'Enter Values' beneath the expense name.
Then simply type the new amount into the months where you need to change the data and click save.
Now you can see the updated figures of discounted rent for 2 months in summer: