Yes, you can assign an individual learner lessons outside of their assigned training plan. This can be done a few ways:
You can assign it to them navigating to Admin > Users and clicking on the learner's email address > "Assign Lessons"
You can also assign content by clicking "Edit" under the Admin > Users and then "Assign Lessons"
If a learner is a member of a team, you can also assign lessons from the Teams feature users list.