Unfortunately, this is not currently possible.
However, a workaround is to enable the Must Require A Receipt option on the Expense Type, but then on the VAT Receipt Options (in Settings, General, VAT Receipt Options), create a new VAT Receipt Option and enable the Allow Receipt Override option:
Create a receipt type for "No receipt" and tick the Allow Receipt Override.
Mark as inactive the existing VAT Receipt Option called “I do not have a receipt”
In this scenario, when the user then uses this type of Expense Type, and then selects the new "No receipt" option, they will have to enter a comment instead of attaching a receipt. If they have selected a receipt type that does not have the Allow Override option ticked, such as "I have a VAT receipt", then they will have to upload a receipt.