"Unit supplement" on salary

Our employees receive a daily travel allowance based on the distance between their workplace and the project site.

Driving zone 1: NOK 150

Driving zone 2: NOK 250

Travel zone 3: NOK 400

It would be great if this could be added under extra activities (or in some other way) by employees when they drive for hours that are not related to their working hours, as a pure monetary allowance.