Yes, you can do it.
The procedure is the same, both for creating a Default Fleet or a Default Driver.
First you need to create a Subfleet and you can do it from the Route Plans tab, under Drivers in the lower part of the screen.
By selecting Save as a Subfleet you create a Subfleet with all the Drivers currently assigned to that Plan. You can do this even if you have one Driver only. Please note that Driver(s)' settings will correspond to the settings for the current plan, so they may differ from Default Settings defined under the tab My Fleet.
After creating and saving a Subfleet, you can make it the Default Subfleet which will be automatically assigned to all future plans.
In order to do so, click on Select Drivers, then on Subfleets and Defaults. Saved Subfleets will be shown and by selecting Make Default you will have the Subfleet/Driver automatically added to all future plans.