I think you can't define the boundary by using circle, but you can change the shape of the boundary which you defined by using spline to a Circle, Horizontal/Vertical ellipse, spline and ASME 94 Circle. Have a look at the detailed view edit window.
Anywhere on the drawing(not on the view) hold down right mouse botton.When menu comes up click on the properties.In file prperties menu click Drawing Options.Under Options control views find click on detail_view_boundary_type.On the bottom of the screen in Value box click down arrow to open all options and click on circle.Safe drawing setup file and add to your config file drawing_setup_file so next time when you open Pro-e it will be there automaticly.