You sure can! Simply add sites to your trip as you visit them rather than before.
WikiTip:
If you're using trip planner like this, you can use the 'current location' option to quickly add custom places (i.e. those which aren't normally found on WikiCamps, like cafes) to your trip plan. Find out how here.
WikiCamps. By campers, for campers.
Article relates to Android, Apple, and Windows devices