Can a person be both product owner and member of a team?
No, right now, a person cannot have multiple roles in the same project. He can be either team member, project owner, or stakeholder, but not multiple of these in the same project.
However, the roles only affect what you see in the Overview tab, People tab, and in the lists of team members various places. So, you can just assign the most important role to the person - either product owner, stakeholder, or team member - and everything will work fine everywhere. The roles do not affect sprint planning, burndown charts, etc.